SKELETON  . In most animals, and indeed in See also:

• PLANTS

plants, the shape could not be maintained without a thickening and hardening of certain parts to See also:

• FORM (Lat. forma)

form a support for the whole . These hardened parts are called the skeleton (Gr. ociXXco, I dry), because they dry up and remain after the See also:

• REST (O. Eng. rest, reste, bed, cognate with other Teutonic forms, e.g. Ger. Rast, Riiste, rest, and probably Gothic Rasta, league, i.e. resting or stopping place)

rest of the See also:

• BODY

body has disappeared . In animals the skeleton is usually, and in higher animals always, rendered more rigid and permanent by the See also:

• DEPOSIT (Lat. depositurn, from deponere, to lay down, to put in the care of)

deposit in it of See also:

• LIME
• LIME (O. Eng. lim, Lat. limes, mud, from linere, to smear)

lime salts, thus leading to the formation of See also:

• BONE (a word common in various forms to Teutonic languages, in many of which it is confined to the shank of the leg, as in the German Bein)
• BONE, HENRY (1755-1834)

bone . Sometimes, as in most of the See also:

• LOWER

lower or invertebrate animals, the skeleton is on the See also:

• SURFACE

surface and thus acts as a See also:

• PROTECTION

protection as well as a framework . This is known as an exoskeleton . In the higher or vertebrate animals there is an See also:

• INTERNAL

internal or endoskeleton and the exoskeleton is either greatly modified or disappears . The following descriptive See also:

• ACCOUNT
• ACCOUNT (through O. Fr. acont, Late Lat. comptum, cornputare, to calculate)

account is divided into (1) axial, or skeleton of the See also:

• TRUNK (Fr. tronc, Lat. truncus, cut off, maimed)

trunk, (2) appendicular or skeleton of the limbs, (3) See also:

• SKULL

skull, (4) visceral skeleton, or those parts which originally form the gill supports of See also:

• WATER

water breathing vertebrates, (5) the exoskeleton, which is considered under the heading SKIN AND EXOSKELETON . These divisions, although they seem logical, cannot in practice be strictly adhered to, especially in the See also:

• CASE
• CASE, JOHN (d. 1600)

case of the visceral skeleton, because doing so would involve, among other things, separating the description of the upper See also:

• JAW (Mid. Eng. jawe, jowe and geowe, O. Eng. cheowan, connected with " chaw " and " chew," and in form with " jowl ")

jaw from that of the rest of the skull . For the microscopical structure of bone see CONNECTIVE TISSUES . Axial . The axial skeleton, from a strictly scientific point of view, should comprise a See also:

• GOOD, JOHN MASON (1764-1827)

good See also:

• DEAL

deal of the skull as well as the See also:

• SPINAL

spinal See also:

• COLUMN (Lat. columna)

column, ribs and See also:

• BREAST (a word common to Teutonic languages, cf. the Ger. Brust, possibly connected with an O. Sax. brustian, to bud)

breast bone, but, as the skull (q.v.) is dealt with in a See also:

• SEPARATE

separate See also:

• ARTICLE (from Lat. articulus, a joint)

article, the three latter structures alone are dealt with here .

The SPINE, SPINAL or VERTEBRAL COLUMN, chine or backbone in See also:

• MAN
• MAN, ISLE OF (anc. Mona)

man consists of a number of superimposed bones which are named vertebrae, because they can move or turn somewhat on spine. each other . It lies in the See also:

• MIDDLE

middle of the back of the See also:

• NECK (O. Eng. hnecca; the word appears in many Teutonic languages; cf. Dutch ride, Ger. Nacken; in O. E. the common word was heals; cf. Ger. Hals)

neck and trunk; has the cranium at its See also:

• SUMMIT

summit; the ribs at its sides, which in their turn support the upper limbs; whilst the See also:

• PELVIS (Lat. for " basin," cf. Gr. 7rEX?a )

pelvis, with the lower limbs, is jointed to its lower end . The spine consists in an adult of twenty-six bones, in a See also:

• YOUNG
• YOUNG, A
• YOUNG, ARTHUR (1741-1820)
• YOUNG, BRIGHAM (1801-1877)
• YOUNG, CHARLES MAYNE (1777–1856)
• YOUNG, EDWARD (1683–1765)
• YOUNG, JAMES (1811-1883)
• YOUNG, THOMAS (1773-1829)

young See also:

• CHILD
• CHILD, FRANCIS JAMES (1825-1896)
• CHILD, L
• CHILD, LYDIA MARIA (1802-1880)
• CHILD, SIR FRANCIS (1642-1713)
• CHILD, SIR JOHN (d. 1690)
• CHILD, SIR JOSIAH (163o - 1699)

child of See also:

• THIRTY

thirty-three, certain of the bones in the spine of the child becoming ankylosed or blended with each other in the adult . These blended bones lose their mobility and are called false vertebrae; whilst those which retain their mobility are the true vertebrae . The bones of the spine are arranged in See also:

• GROUPS
• GROUPS, THEORY

groups, which are named from their position—vertebrae of the neck or cervical; of the See also:

• CHEST (Gr. Kiarl, Lat. cista, O. Eng. cist, test, &c.)

chest, thoracic, formerly called dorsal; of the loins, lumbar; of the pelvis, sacral; and of the tail, See also:

• COCCYGEAL

coccygeal or caudal; and the number of vertebrae in each See also:

• GROUP

group may be expressed in a See also:

• FORMULA
• FORMULA (Lat. diminutive of forma, shape, pattern, &c., especially used of rules of judicial procedure)

formula . In man the formula is as follows: C7Th72L5S5Coc, =33 bones, as seen in the child; but the five sacral vertebrae fuse together into a single bone—the sacrum—and the four coccygeal into the single coccyx . Hence the sacrum and coccyx of the adult are the false, whilst the lumbar, dorsal and cervical are the true vertebrae . The vertebrae are irregularly-shaped bones, but as a See also:

• RULE

rule have certain characters in See also:

• COMMON

common . Each possesses a body and an See also:

• ARCH
• ARCH, JOSEPH (1826– )

arch, which enclose a See also:

• RING (O.E. hring; a word common to Teutonic languages; and probably cognate with the Lat. circus, Gr. KtpKOS or KpLKOS, Skt'. chakra, wheel, circle, cf. also " harangue "); in art, a band of circular shape of varying sizes, made of any material and used f

ring, with certain processes and notches . The body, or centrum, is a See also:

• SHORT, FRANCIS JOB (1857– )

short See also:

• CYLINDER (Gr. KvAwSpos, from KvXivaety, to roll)

cylinder, which by its upper and lower surfaces is connected by means of fibrocartilage with the bodies of the vertebrae immediately above and below . The collective See also:

• SERIES (a Latin word from serere, to join)

series of vertebral bodies forms the See also:

• GREAT

great column of the spine . The arch, also called neural arch, because it encloses the spinal marrow or See also:

• NERVOUS

nervous See also:

• AXIS (Lat. for " axle ")

axis, springs from the back of the centrum, and consists of two symmetrical halves See also:

• UNITED

united behind in the middle See also:

• LINE

line .

Each See also:

• HALF

half has an anterior See also:

• PART

part or pedicle, and a posterior part or lamina . The rings collectively form the spinal See also:

• CANAL
• CANAL (from Lat. canalis, " channel " and " kennel " being doublets of the word)

canal . The processes usually See also:

• SPRING (from " to spring," " to leap or jump up," " burst out," O. Eng., springau, a common Teut. word, cf. Ger. springer, possibly allied to Gr. QnrFpxecOac, to move rapidly)

spring from the arch . The spinous See also:

• PROCESS

process projects backward from the junction of the two laminae, and the collective series of these processes gives to the entire column the spiny See also:

• CHARACTER (Gr. xapareri7p, from xap&crew, to scratch)

character from which has arisen the See also:

• TERM

term spine, applied to it . The transverse processes project out-See also:

• WARD
• WARD, ADOLPHUS WILLIAM (1837- )
• WARD, ARTEMUS
• WARD, EDWARD MATTHEW (1816-1879)
• WARD, ELIZABETH STUART PHELPS (1844-1911)
• WARD, JAMES (1769--1859)
• WARD, JAMES (1843– )
• WARD, JOHN QUINCY ADAMS (1830-1910)
• WARD, LESTER FRANK (1841– )
• WARD, MARY AUGUSTA [MRS HUMPHRY WARD]
• WARD, WILLIAM (1766-1826)
• WARD, WILLIAM GEORGE (1812-1882)

ward, one from each See also:

• SIDE
• SIDE (mod. Eski Adalia)

side of the arch . The articular processes project, two upward and two downward, and are for connecting adjacent vertebrae together . The notches, situated on the upper and lower See also:

• BORDERS, THE

borders of the pedicles, form in the articulated spine the intervertebral foramina through which the nerves pass out of the spinal canal . The vertebrae in each group have characters which specially distinguish them . In man and all mammals, with few exceptions, whatever be the length of the neck, the cervical vertebrae cervical are seven in number . In man the body of a cervical vertebrae. vertebra is comparatively small, and its upper surface is transversely See also:

• CONCAVE

concave; the arch has See also:

• LONG, GEORGE (1800-1879)
• LONG, JOHN DAVIS (1838– )

long and obliquely sloping laminae; the ring is large and triangular; the spine is short, bifid, and See also:

• HORIZONTAL

horizontal ; the transverse process consists of two bars of bone, the anterior springing from the side of the body, the posterior from the arch, and uniting externally to enclose a foramen (vertebrarterial) through which, as a rule, the vertebral artery passes; the articular processes are See also:

• FLAT (a modification of O. Eng. flet, an obsolete word of Teutonic origin, meaning the ground beneath the feet)

flat and oblique, and the upper pair of notches are deeper than the lower . The first, second and seventh cervical vertebrae have. characters which specially distinguish them . The first, or See also:

• ATLAS

atlas, has no body or spine: its ring is very large, and on each side of the ring is a thick See also:

• MASS (O.E. maesse; Fr. messe; Ger. Messe; Ital. messa; from eccl. Lat. missa)
• MASS, IN

mass of bone, the lateral mass, by vi vi Skeleton .

C7 The cervical vertebrae . D,2 The thoracic . L5 The lumbar . S5 The sacral . Coc4The coccygeal . CC The series of twelve ribs on one side . Ps The presternum . Ms The meso-sternum . Xs The xiphisternum . The dotted line VV represents the See also:

• VERTICAL (from Lat. vertex, highest point)

vertical axis of the spine . Axial which it articulates with the occipital bone above and the second vertebra below . The second vertebra, axis, or Vertebra dentata, has its body surmounted by a thick, tooth-like odontoid process, which is regarded as the body of the atlas displaced from its proper vertebra and fused with the axis .

This process forms a See also:

• PIVOT (Fr. pivot; probably connected with Ital. pivolo, peg, pin, diminutive of piva, pipa, pipe)

pivot See also:

• ROUND (O. Fr. rond, Lat. rotundus, the Fr. is the source also of Du. rond; Ger., Swed., Dan. and Nor. rond)

round which the atlas and See also:

• HEAD (in 0. Eng. heafad; the word is common to Teutonic languages; cf. Dutch hoofd, Ger. Haupt, generally taken to be in origin connected with Lat. caput, Gr. KerbOvi7)
• HEAD, SIR EDMUND WALKER, BART
• HEAD, SIR FRANCIS BOND, BART

head move in turning the head from one side to the other; the spine is large, thick and deeply bifid . The seventh, called Vertebra prominens, is distinguished by its long prominent spine, which is not bifid, and by the small See also:

• SIZE

size of the foramen at the See also:

• ROOT (late O.E. rot, adopted from Scand., cf. Norw. and Swed. rot, Dan. rod; the true O.E. word was wyrt, plant, represented in Ger. Wurz or Wurzel; the ultimate root is the same in both words, and is seen in Lat. radix)
• ROOT, ELIHU (1845– )

root of the transverse process . In the human spine the distinguishing character of all the cervical vertebrae is the foramen at the root of the trans-See also:

• VERSE (from Lat. versus, literally a line or furrow drawn by turning the plough, from vertere, and afterwards signifying an arrangement of syllables into feet)

verse process . The thoracic vertebrae, formerly called dorsal, are twelve i.1 number Thracic in the human spine . They verotebrae. are intermediate in size and position to the cervical and lumbar vertebrae, and are all distinguished by having one or two smooth surfaces on each side of the body for See also:

• ARTICULATION (from Lat. articulare, to divide into joints)

articulation with the head of one or two ribs . The arch is short and with imbricated laminae; the ring is nearly circular; the spine is oblique, elongated and See also:

• BAYONET

bayonet-shaped; the transverse processes are directed back and out, not bifid, and with an articular surface in front for the tubercle of a See also:

• RIB (from 0. Eng. ribb; the word appears' in many Teutonic languages, cf. Ger. Rippe, Swed. reb)

rib; and the articular processes are flat and nearly vertical . The first, twelfth, See also:

• ELEVENTH

eleventh, tenth and sometimes the ninth, thoracic vertebrae are distinguished from the rest . The first is in shape like the seventh cervical, but has no foramen at the root of the transverse process, and has two articular facets on each side of the body; the ninth has sometimes only one facet at the side of the body; the tenth, eleventh, and twelfth have invariably only a single facet on the side of the body, but the eleventh and twelfth have stunted transverse processes, and the twelfth has its lower articular processes shaped like those of a lumbar vertebra . The lumbar vertebrae in man are five in number . They are the lowest Lumbar of the true vertebrae, and vertebrae. also the largest, especially in the centrum . The arch has short and deep laminae; the ring is triangular; the spine is massive and See also:

• HATCHET (adapted from the Fr. hachette, diminutive of hache, axe, hacker, to cut, hack)

hatchet-shaped; the trans-verse processes are long and pointed ; the articular are thick and strong, the See also:

• SUPERIOR

superior pair concave, the inferior See also:

• CONVEX

convex, and the inferior notches, as in the thoracic vertebrae, are deeper than the superior . In the lumbar vertebrae and in the lower thoracic an See also:

• ACCESSORY

accessory process projects from the See also:

• BASE

base of each transverse process, and a mammillary tubercle from each superior articular process .

The fifth lumbar vertebra has its body much deeper in front than behind and its spine is less massive . The sacrum is composed of five originally separate vertebrae fused Sacrum. into a single bone . It forms the upper and back See also:

• WALL (O. Eng. weal, weall, Mid. Eng. wal, wane, adapted from Lat. vallum, rampart; the original O. Eng. word for a wall was wag or tenth)
• WALL, RICHARD (1694-1778)

wall of the pelvis, is triangular in form, and possesses two surfaces, two borders, a base, and an See also:

• APEX

apex . The anterior or pelvic surface is concave, and is marked by four transverse lines, which indicate its See also:

• ORIGINAL

original subdivision into five bones, and by four pairs of foramina, through which are transmitted the anterior sacral nerves . Its posterior surface is convex; in the middle line are four spines, because in the last sacral vertebra the spinal canal is not closed behind . On each side of these are two rows of tubercles, the inner of which are the conjoined articular and mammillary processes, the See also:

• OUTER

outer the transverse processes of the originally distinct vertebrae . Between these rows four pairs of foramina are found transmitting the posterior sacral nerves from the sacral canal, which extends through the bone from base to near the apex, and forms the lower end of the spinal canal . By its borders the sacrum is articulated with the haunch-bones—by its base with the last lumbar vertebra, by its apex with the coccyx . The human sacrum is broader in proportion to its length than in other mammals; this great breadth gives solidity to the lower part of the spine, and, conjoined with the size of the lateral articular surfaces, it permits a more perfect junction with the haunch-bones, and is correlated with the erect position . Owing to the need in woman for a wide pelvis, the sacrum is broader than in man . (For details see A . M .

See also:

• PATERSON
• PATERSON, ROBERT (1715-1801)
• PATERSON, WILLIAM (1658-1719)

Paterson, " The Human See also:

• SAC

Sac-See also:

• RUM
• RUM (according to Skeat, a corruption of Malay brum or bram; the adjective " rum," i.e. " queer," being a distinct word, in Gipsy rem)

rum," Sci . Trans . R . See also:

• DUBLIN

Dublin See also:

• SOC

Soc. vol. v. See also:

• SER

ser . 2.) The coccyx consists of four or five vertebrae in the human spine though the last one is sometimes suppressed . It is the rudimentary tail, but instead of projecting back, as in mammals generally, is curved forward, and is not visible externally, c"cY''. an arrangement which is also found in the anthropoid apes and in See also:

• HOFFMANN, AUGUST HEINRICH (1798–1874)
• HOFFMANN, ERNST THEODOR WILHELM (1776-1822)
• HOFFMANN, JOHANN JOSEPH (1805-1878)

Hoffmann's See also:

• SLOTH

sloth . Not only is the tail itself rudimentary in man, but the vertebrae of which it is composed are small, and represent merely the bodies and transverse processes of the true vertebrae . As there are no See also:

• ARCHES, COURT OF

arches, the ring is not formed, and the spinal canal does not extend, therefore, beyond the See also:

• FOURTH

fourth piece of the sacrum . The first coccygeal vertebra, in addition to a body, possesses two processes or horns, which are the superior articular processes . The human spine is more See also:

• UNIFORM

uniform in length in persons of the same See also:

• RACE

race than might be supposed from the individual See also:

• DIFFERENCES, CALCULUS OF (Theory of Finite Differences)

differences in stature, the variation in the height of the body in adults being due chiefly to differences in the length of the lower limbs . The See also:

• AVERAGE

average length of the spine is 28 in.; its widest part is at the base of the sacrum, from which it tapers down to the tip of the "coccyx . It diminishes also in breadth from the base of the sacrum upwards to the region of the neck .

Owing to the See also:

• PROJECTION

projection of the spines behind and the transverse processes on each side, it presents an irregular outline on those aspects; but in front it is more uniformly rounded, owing to the convex form of the antero-lateral surfaces of the bodies of its respective vertebrae . In its See also:

• GENERAL
• GENERAL (Lat. generalis, of or relating to a genus, kind or class)

general See also:

• CONTOUR

contour two series of curves may be seen, an antero-posterior and a lateral . The antero-posterior is the more important . In the See also:

• INFANT (in early forms enfaunt, enfant, through the Fr. enfant, from Lat. infans, in, not, and fans, the present participle of fari, to speak)

infant at the See also:

• TIME (0. Eng. Lima, cf. Icel. timi, Swed. timme, hour, Dan. time; from the root also seen in " tide," properly the time of between the flow and ebb of the sea, cf. O. Eng. getidan, to happen, " even-tide," &c.; it is not directly related to Lat. tempus)
• TIME, MEASUREMENT OF
• TIME, STANDARD

time of See also:

• BIRTH (a word common in various forms to Teutonic languages from the root of the verb " to bear ")

birth the sacrococcygeal part of the spine is concave forward, but the rest of the spine, except a slight forward concavity in the series of thoracic vertebrae, is almost straight . When the infant begins to sit up in the arms of its See also:

• NURSE (a shortened form of the earlier " nourice," adapted through the French from Lat. nutrix, nu/rite, to nourish)

nurse, a convexity forward in the region of the neck appears, and subsequently, as the child learns to walk, a convexity forward in the region of the loins . Hence in the adult spine a series of convexo-concave curves are found, which are alternate and mutually dependent, and are associated with the erect attitude of man . A lateral See also:

• CURVE (Lat. curvus, bent)

curve, convex to the right, opposite the third, fourth, and fifth thoracic vertebrae, with compensatory curve convex to the See also:

• LEFT

left immediately above and below, is due apparently to the much greater use of the muscles of the right See also:

• ARM (a common Teutonic word; the Indo-European root is ar, to join or fit; cf. the Lat. armus, shoulder, and the plural word arma, weapons, Gr. apphs, joint, and the reduplicated apapilKSV, to join)

arm over those of the left, See also:

• DRAWING

drawing the spine in that region somewhat to the right . In disease of the spine its natural curvatures are much increased, and the deformity known as humpback is produced . As the spine forms the central part of the axial skeleton, it acts as a column to support not only the See also:

• WEIGHT

weight of the body, but of all that can be carried on the head, back and in the upper limbs: by its transverse and spinous processes it serves also to give See also:

• ATTACHMENT

attachment to numerous muscles, and the transverse processes of its thoracic vertebrae are also for articulation with the ribs . The See also:

• THORAX (Gr. BWpaE, breastplate, also the part of the body covered by it)

THORAX, PECTUS, or CHEST is a cavity or enclosure the walls of which are in part formed of bone and See also:

• CARTILAGE (Lat. cartilage, gristle)

cartilage . Its skeleton consists of the sternum in front, the twelve thoracic Tboraa. vertebrae behind, and the twelve ribs, with their corre- sponding cartilages, on each side . The sternum or breast bone is an elongated bone which inclines downward and forward in the front wall of the chest .

It consists of three parts—an upper, called manubrium or presternum; Sternum. a middle, the See also:

• GLADIOLUS

gladiolus or mesosternum; and a lower, the ensiform process or xiphisternum . Its anterior and posterior surfaces are marked by transverse lines, which indicate not only the subdivision of the entire bone into three parts, but that of the mesosternum into four originally distinct segments . Each lateral border of the bone is marked by seven depressed surfaces for articulation with the seven upper ribs: at each side of the upper border of the presternum is a sinuous depression, where the clavicle, a bone of the upper See also:

• LIMB

limb, articulates with this bone of the axial skeleton . The xiphisternum remains cartilaginous up to a See also:

• LATE

late See also:

• PERIOD
• PERIOD (Gr. irepioSos, a going or way round, circuit, aepi, round, and &Sis, way, road)

period of See also:

• LIFE

life, and from its pointed form has been named the ensiform cartilage . The ribs or costae, twenty-four in number, twelve on each side of the thorax, consist not only of the bony ribs, but of a See also:

• BAR
• BAR (0. Fr. barre, Late Lat. barra, origin unknown)
• BAR, CONFEDERATION OF
• BAR, FRANCOIS DE (1538-1606)
• BAR, THE

bar of cartilage continuous with the anterior end of each bone, costae. called a costal cartilage, so that they furnish examples of a cartilaginous skeleton in the adult human body; in aged persons these cartilages usually become converted into bone . The upper seven ribs are connected by their costal cartilages to the side of the sternum, and are called sternal or true ribs; the lower five do not reach the sternum, and are, named a-sternal or false, and of these the two lowest, from being comparatively unattached in front, are called See also:

• FREE

free or floating . Another and pethaps more useful See also:

• CLASSIFICATION (Lat. classis, a class, probably from the root cal-, cla-, as in Gr. icaMce, clamor)

classification is to speak of the first seven ribs as vertebro-sternal, the next three as vertebro-costal, and the last two as vertebral . All the ribs are From See also:

• ARTHUR (Fr. Arius)
• ARTHUR, CHESTER ALAN (1830 – 1886)

Arthur See also:

• THOMSON, JAMES (1822-1892)
• THOMSON, JAMES (1834-1882)
• THOMSON, JAMES (r700-1748)
• THOMSON, JOHN (1778-184o)
• THOMSON, JOSEPH (1858-1895)
• THOMSON, SIR CHARLES WYVILLE (1830-1882)
• THOMSON, THOMAS (1773-1852)
• THOMSON, WILLIAM (1819-189o)

Thomson, Cunning-See also:

• HAM

ham's See also:

• TEXT (Lat. textum, woven fabric, from texere, to weave)

Text-See also:

• BOOK

Book of See also:

• ANATOMY
• ANATOMY (Gr. avaroyil, from ava-silo c', to cut up)

Anatomy . Fic . 2.—Vertebral Column as seen from behind . articulated behind to the thoracic vertebrae, and as they are sym- allows the passage of the See also:

• WINDPIPE

windpipe, gullet, large See also:

• VEINS

veins and nerves metrical on the two sides of the body, the ribs in any given See also:

• ANIMAL
• ANIMAL (Lat. animalis, from anima, breath, soul)

animal into the chest, and of several large See also:

• ARTERIES (Gr. ap-rnpia, probably from a"spew, to raise, but popularly connected by the ancients with ai7P, air)

arteries out of the chest into the are always twice as numerous as the thoracic vertebrae in that neck . The base or lower boundary of the cavity is much larger than animal .

They form a series of osseocartilaginous arches, which the upper, slopes downward and backward, and is occupied by the See also:

• DIAPHRAGM (Gr. Scat/pay,aa, a partition)

diaphragm, a muscle which separates the chest from the cavity of the See also:

• ABDOMEN (a Latin word, either from abdere, to hide, or from a form adipomen, from adeps, fat)

abdomen . The transverse See also:

• DIAMETER (from the Cr. &a, through, µErpov, measure)

diameter is greater than the anteroposterior, and the antero-posterior is greater laterally, where the lungs are lodged, than in the mesial See also:

• PLANE

plane, which is occupied by the See also:

• HEART
• HEART, LUNG

heart . See also:

• EMBRYOLOGY

Embryology.—The first See also:

• APPEARANCE (from Lat. apparere, to appear)

appearance of any stiffening of the embryo is the formation of the notochord, which in the higher vertebrates is a temporary structure and is not converted into cartilage or bone . It also differs from the bony skeleton in that it is derived from the entoderm or inner of the three,layers of the embryo while the bony skeleton is formed from the mesoderm or middle layer and, just as the entoderm is an older layer of the embryo than the mesoderm, so the notochord or entodermal skeleton precedes, both in embryology and in phylogeny or See also:

• COMPARATIVE

comparative anatomy, the bony mesodermal skeleton . In the accompanying figure (fig . 4) the notochord is seen in See also:

• SECTION
• SECTION (Lat. sectio, cutting, secare, to cut)

section fully formed and lying between the entoderm and the neural canal . Its first formation is at an earlier period than this, before the neural groove has closed into a canal, and it appears at first as an tT and groove from the most dorsal part of the entoderm in what will later on be the cervical region of the embryo . The groove, by the See also:

• UNION
• UNION (known locally as Union Hill and officially as Town of Union)

union of its edges, becomes a See also:

• TUBE (Lat. tuba)

tube, sometimes spoken of as the chordal tube, but the cavity of this is soon obliterated by the growth of its cells, so that a solid elastic See also:

• ROD (O.E. rodd, probably related to Norw. rudda, stick, rodda, stake)
• ROD, EDOUARD (1857-1910)

rod is formed which grows forward as far as the pituitary region of the skull and backward to where the end of the coccyx will be . While the development of the notochord is going on the mesoderm on each side of it is dividing itself into a series of masses called mesodermic somites (see fig . 4, PS) or protovertebrae . This process begins in the cervical region and proceeds forward and backward until thirty-eight pairs have been formed for the neck and trunk and probably four extra ones for the occipital region of the skull . Each of these somites consists of three parts: that nearest the surface ectoderm is the cutaneous lamella (fig .

4, CL) . Deep to this and separated in the earlier-formed somites by a space is the muscle layer (fig . 4, ML) while deepest of all and nearest the See also:

• NERVE (Lat. nervus, Gr. vevpov, a bowstring)

nerve See also:

• CORD (derived through the Fr. corde, from the Lat. chorda, Gr. xop(5rt, the string of a musical instrument)

cord and notochord is the scleratogenous layer (fig . 4, SL) . It is this layer which gradually meets its See also:

• FELLOW

fellow of the opposite side and encloses the nerve cord and the notochord in continuous tubes of mesodermal See also:

• TISSUE (Fr. tissu, tissue, participle of tisser, Lat. texere, to weave)

tissue, thus forming the membranous vertebral column, which is perforated forthe exit of the spinal nerves, but the intervals between the successive mesodermic somites are still marked by the tissue being rather denser there . The next See also:

• STAGE (Fr. 6/age; from Lat. stare, to stand)

stage is that of chondrification or the See also:

• CONVERSION (Lat. conversio, from convertere, to turn or ,change)

conversion into cartilage of each segment of the membranous vertebral column surrounding the notochord . In this way the bodies of the cartilaginous vertebrae are formed and each of these is segmental, that is, it corresponds to a muscle segment and a spinal nerve . The cartilaginous neural arch, however, which surrounds the nerve cord is intersegmental and is formed in the denser fibrous tissue which separates each somite from the next . This also applies to the cartilaginous ribs which appear in the fibrous intervals (myo- SB NC CC CC Central canal . ML See also:

• MUSCULAR

Muscular layer of meso- SG Spinal ganglion . CL Cutaneous lamella dermic somite . SL Scleratogenous layer of protovertebral N Notochord .

of protovertebral somite . NC Neural See also:

• CREST
• CREST (Lat. crista, a plume or tuft)

crest . somite . CO Coelom . PA See also:

• PRIMITIVE

Primitive aorta . SoM Somatic mesoderm . EC Ectoderm . PS Mesodermic somite . SoP Somatopleure . EN Ento.lerm . SB Spongioblast . SpM Splanchnic mesoderm .

GC Germinal See also:

• CELL (from Lat. cella, probably from an Indo-European kal —seen in Lat. celare, to hide; another suggestion connects the word with Lat. cera, wax, taking the original meaning to refer to the honeycomb)

cell . SC Spinal cord . SpP Splanchnopleure . commata) between the muscle plates (myotomes), and so it is easy to realize that each typical rib must articulate with the bodies of two adjacent vertebrae, but with the neural arch, through its trans-verse process, of only one . The intersegmental tissue between the bodies of the vertebrae becomes the intervertebral discs and in the centre of these a pulpy From Arthur Thomson, See also:

• CUNNINGHAM, ALEXANDER (c.1655-173o)
• CUNNINGHAM, ALLAN (1784-1842)
• CUNNINGHAM, WILLIAM (1805-1861)

Cunningham's Text-Book of Anatomy . extend more or less perfectly around the sides of the chest . A rib is an elongated bone, and as a rule possesses a head, a neck, a tubercle and a See also:

• SHAFT
• SHAFT (O. Eng., sceaft, from scafan, to shave; the word is common to Teutonic languages)

shaft . The head usually has two articular surfaces, and is connected to the side of the body of two adjacent thoracic vertebrae; the neck is a constricted part of the bone, uniting the head to the shaft; the tubercle, See also:

• CLOSE
• CLOSE (from Lat. clausum, shut)
• CLOSE, MAXWELL HENRY (1822-1903)

close to the junction of the shaft and neck, is the part which articulates with the transverse process of the vertebra . The shaft is compressed, possesses an inner and outer surface, and an upper and lower border, but from the shaft being somewhat See also:

• TWISTED

twisted on itself, the direction of the surfaces and borders is not uniform throughout the length of the bone . The ribs slope from their attachments to the spine, at first outward, downward and back-ward, then downward and forward, and where the curve changes from the backward to the forward direction an See also:

• ANGLE (from the Lat. angulus, a corner, a diminutive, of which the primitive form, angus, does not occur in Latin; cognate are the Lat. angere, to compress into a bend or to strangle, and the Gr. ayKOS, a bend; both connected with the Aryan root ank-, to

angle is formed on the rib . The angle and the tubercle are at the same See also:

• PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

place in the first rib and in each succeeding rib the angle is a little farther from the tubercle than in the last . The first, tenth, eleventh and twelfth ribs articulate each with only one vertebra so that there is only one surface on the head .

The surface of the first rib which is not in contact with the See also:

• LUNG

lung is directed upward, forward and outward while that of the second rib is much more outward; the eleventh and twelfth ribs are rudimentary, have neither neck nor tubercle, and are pointed anteriorly . The ribs are by no means uniform in length: they increase from the first to the seventh or eighth, and then diminish to the twelfth; the first and twelfth are therefore the shortest ribs . The first and second costal cartilages are almost horizontal, directed upward and inward . In its general form the chest may be likened to a See also:

• BARREL (a word of uncertain origin common to Romance languages; the Celtic forms, as in the Gaelic baraill, are derived from the English)

barrel which is wider below than above . It is rounded at the sides and flattened in front and behind, so that a man can See also:

• LIE, JONAS LAURITZ EDEMIL (1833—1908)
• LIE, MARIUS SOPHUS (1842–1899)

lie either on his back or his belly . Its upper opening slopes downward and forward, is small in size, and From See also:

• ALFRED

Alfred H . Young and Arthur See also:

• ROBINSON, EDWARD (1794–1863)
• ROBINSON, HENRY CRABB (1777–1867)
• ROBINSON, JOHN (1575–1625)
• ROBINSON, JOHN (1650-1723)
• ROBINSON, JOHN THOMAS ROMNEY (1792–1882)
• ROBINSON, MARY [" Perdita "] (1758–1800)
• ROBINSON, SIR JOHN BEVERLEY, BART
• ROBINSON, SIR JOSEPH BENJAMIN (1845– )
• ROBINSON, THEODORE (1852-1896)

Robinson, Cunningham's Text-Book of Anatomy . but the others are rip differentiation of mass is found which contains some remnants of the notochord. but the ventral ends of the ninth and sometimes the eighth probably Elsewhere this structure is pressed out of existence and there is no remain as the xiphisternum, indeed a fibrous See also:

• BAND

band is sometimes seen further use for it when the cartilaginous vertebrae are once formed. joining the caudal end of that structure to the ninth rib . The See also:

• FUSION

fusion One other series of structures must be mentioned though they do not of the two parallel bars begins at their cephalic ends and sometimes is 8 19 25 24 27 From Arthur Thomson, Cunningham's Text-Book of Anatomy . Centre for summit of odontoid See also:

• PRO

pro centres appear first in each half of the neural arches See also:

• CESS (a shortened form of " assess "; the spelling is due to a mistaken connexion with " census ")

cess; appears 3rd to 5th See also:

• YEAR

year, fuses of the vertebrae and a little later (tenth See also:

• WEEK (from A.S. wicu, Germanic *wikon, probably=change, turn)

week) 8th to 12th year. See also:

• DOUBLE
• DOUBLE (from the Mid. Eng. duble, the form which gives the present pronunciation, through the Old Er. duble, from Lat. duplus, twice as much)

double centres are deposited in the centra though Appears about 5th or 6th See also:

• MONTH
• MONTH (a common Teutonic word, cf. Ger. Mond, Du. maand, Dan. maaned, &c., and cognate with Lat. mensis, Gr. µi7v, &c., in other branches of the Indo-Germanic family; all ultimately from the root seen in the word for the moon in nearly all those languages

month; these are so close together and fuse so rapidly that interrupted toward the caudal end, thus leading to cleft or perforate sternum . At the cephalic end of each sternal bar, close to the place where the clavicles articulate, is an imperfectly separated patch of cartilage which usually fuses completely with the'presternum, though sometimes it remains distinct and may later acquire a separate centre of ossification and so form a separate episternal bone on each side . If the sternum is to be regarded as the fused ventral ends of the thoracic ribs, the episternal elements are probably the remnants of the ventral ends of the seventh cervical ribs .

The question of the morphological meaning of the sternum and surrounding parts cannot be settled entirely by a study of their development even when combined with what we know of their comparative anatomy or phylogeny . See also:

• PROFESSOR (the Latin noun formed from the verb profiteri, to declare publicly, to acknowledge, profess)

Professor A . M . Paterson (The Human Sternum, See also:

• LONDON

London, 1904) takes a different view from the foregoing and regards the sternum as derived from the See also:

• SHOULDER (in O.E. sculder, cognate with Ger. Schulter, Dutch schouder, Swed. skuldra, &c; the root is unknown)

shoulder See also:

• GIRDLE (O. Eng. gyrdel, from gyrdan, to gird; cf. Ger. Gurtel, Dutch gordel, from giirten and gorden; "gird" and its doublet " girth " together with the other Teutonic cognates have been referred by some to the root ghar—to seize, enclose, seen in Gr. )(s

girdle . Te this point of view we shall return in the section on comparative anatomy . The last stage in the development of the axia! skeleton is the ossification of the cartilage; bony 4 5 6 7 8 9 Io II I2 Cervical Vertebra . 2I I Centre for body . 2 Superior epiphysial See also:

• PLATE

plate . 3 Anterior bar of transverse process See also:

• DEVELOPED

developed by lateral See also:

• EXTENSION (Lat. ex, out ; tendere, to stretch)

extension from 23 pedicle . Neuro-central synchondrosis . Inferior epiphysial plate . 24 Lumbar Vertebra .

Body . Superior epiphysial plate . Epiphysis for mammillary process . Epiphysis for transverse process . Epiphysis for spine . Neuro-central synchondrosis . Inferior epiphysial plate . Dorsal Vertebra . 13 Centre for body . 14 Superior epiphysial plate, appears about See also:

• PUBERTY (Lat. pubertas, from pubes, puber, mature, adult)

puberty; unites at 25th year . 15 Neuro-central synchondrosis does not ossify till 5th or 6th year . 16 Appears at puberty; unites at 25th year .

17 Appears at puberty; unites at 25th year . 18 Appears about 6th week . Axis . 19 Centre for transverse process neural arch; appears about week . 20 Synchondroses close about 3rd year . See also:

• PLAY

play any great part in human development . In the intersegmental tissue ventral to each of the intervertebral disks a transverse rod of cells, known as a hypochordal bar, is formed which connects the heads of two opposite ribs . In man the greater number of these either disappear or form the middle fasciculus of the stellate See also:

• LIGAMENT (Lat. ligamentum, from ligare, to bind)

ligament which joins the head of the rib to the intervertebral disk, but in the case of the atlas the rod chondrifies to form the anterior (ventral) arch which is therefore intersegmental, while the segmental body of the atlas, through which the notochord is passing, joins the axis to form the odontoid process . These hypochordal bars are interesting as the last remnant in man of the haemal arch of the vertebrae of fishes (see subsection on comparative anatomy) . In the cervical region the ribs are very short and form the ventral boundary of the foramen for the vertebral artery . They are so short that little See also:

• MOVEMENT

movement occurs between them and the rest of the vertebra, hence no See also:

• JOINTS

joints are formed and the rib See also:

• ELEMENT (Lat. elementum)

element becomes fused with the centrum and transverse process, leaving the vertebrarterial canal between . Sometimes in the seventh cervical vertebra the rib element is much longer and then of course more movement occurs, and instead of fusing with the rest of the vertebra it remains as a separate cervical rib with definite joints .

The sternum is developed according to G . See also:

• RUGE, ARNOLD (18o2—188o)

Ruge by a fusion of the ventral ends of the ribs on each side thus forming two parallel See also:

• LONGITUDINAL

longitudinal bars which chondrify and eventually fuse together in the See also:

• MID

mid line . The anterior seven or sometimes eight ribs reach the sternum, for the tubercle and head . The sternum is ossified by centres which do not appear opposite the attachment of the ribs but alternately with them, so that although the original From Arthur Thomson, Cunningham's Text-Book of Anatomy . cartilaginous structure is probably intersegmental the bony segments are segmental like those of the vertebral centra . As seven ribs articulate with the sternum six centres of ossification between them might be looked for, but there is so little See also:

• ROOM

room between the points of attachment of the See also:

• SIXTH

sixth and seventh ribs that centres do not occur 22 25 26 27 29 30 31 and 8th appears about 5th month. therefore articulates with the developmental neural Atlas. arch instead of the centrum . About the See also:

• AGE (Fr. age, through late Lat. aetaticum, from aetas)

age of Posterior arch and lateral masses puberty secondary centres or epiphyses appear at developed from a single centre on the tips of the transverse and spinous processes and either side, which appears about as thin plates just above and below the body (see e week. fig . 5—2 and 3) . These are fully united by the arch and portion of superior twenty-fifth year . In the lower two cervical verte-Anterior articular surface developed frombrae there is often a separate centre for the part aingl or double centre, appearing corresponding to the rib, while the lumbar have an during 1st year. extra epiphysis for the mammillary process . The atlas has one centre for each side of the dorsal part Dorsal Vertebra. of the arch and one (probably two fused) for the 28 Epiphysis for transverse process; ventral part, which has already been referred to as unites with opposite side 7th to 8th their double nature is often only indicated by their month. See also:

• OVAL (Lat. ovum, egg)

oval or dumb-See also:

• BELL
• BELL, ALEXANDER MELVILLE (1819—1905)
• BELL, ANDREW (1753—1832)
• BELL, GEORGE JOSEPH (1770-1843)
• BELL, HENRY (1767-1830)
• BELL, HENRY GLASSFORD (1803-1874)
• BELL, JACOB (1810-1859)
• BELL, JOHN (1691-178o)
• BELL, JOHN (1763-1820)
• BELL, JOHN (1797-1869)
• BELL, ROBERT (1800-1867)
• BELL, SIR CHARLES (1774—1842)

bell-like appearance . The bone in the Synchondrosis closes from 4th to 6th two halves of the neural arch spreads and fuses in year. the mid dorsal line, and later on joins the ossified inferior epiphysial plate; appears centrum ventral to the facet for the rib .

This point about puberty, unites about 25th of junction remains as a narrow See also:

• STRIP

strip of cartilage for year. a long time and is known as the neuro-central Single or double centre for body; suture or synchondrosis . The head of the rib appears about puberty, unites about a hypochordal bar . In the axis, in addition to the 25th year. See also:

• ORDINARY (med. Lat. ordinarius, Fr. ordinaire)

ordinary centres, there is one for each side of the Epiphysis appears' about puberty; odontoid process and one for the tip (see fig . 5 unites about 25th or 27th year . 20, 21, 22) . The sacral vertebrae have the usual Centre for neural arch on either side; centres, except that the anterior part of the lateral appears about 6th or 7th week, the mass (costal element) has a separate centre and laminae unite from birth to 15th that there are two extra centres on each side of month. the whole sacrum where it articulates with the ilium Centre for body; appears about 6th (see fig . 6) . week, unites with neural arch from The ribs ossify by one See also:

• PRIMARY

primary centre appearing 5th to 6th year. about the sixth week and by secondary ones here as a rule. consequently five centres are found; those for the two higher segments being single while the lower ones are often double . Later on in life a centre for the xiphisternum appears . At birth . At 3 years . From Arthur Thomson, Cunningham's Text-Book of Anatomy .

well as the third segment of the body possesses two centres . 1 Appears about 5th or 6th month; III. segment unites month. with II. about puberty; IV . 2 Appear about 7th month; segment unites with III . unite from 20 to 25. See also:

• EARLY
• EARLY, JUBAL ANDERSON (1816-1894)

early childhood . [later . 3 Appear about 8th or 9th 4 Appears about 3rd_ year or adult salamanders, but usually the intercentral remnants of the notochord are pressed out of existence by the forward growth of the centrum behind it, so that in the adult each vertebra is only concave behind (opisthocoelous) . In the Anura (frogs and toads), on the other See also:

• HAND
• HAND (a word common to Teutonic languages; cf. Ger. Hand, Goth. handus)
• HAND, FERDINAND GOTTHELF (1786-185r)

hand, the centra are usually concave forward (procoelous) and some or the posterior ones become fused into a long delicate bone, the urostyle . The ribs of urodeles have forked vertebral ends and are thus attached to the centrum as well as to the neural arch of a vertebra; this forking is supposed to be homologous with the double ribs of Polypterus already referred to . The sternum as a See also:

• CONSTANT, BENJAMIN (1845-1902)

constant structure first appears in amphibians and is more closely connected with the shoulder girdle than with the ribs, the ventral ends of which, except in the See also:

• SALAMANDER

salamander Necturus, are rudimentary . It is not certain whether it is the homologue of the sternum of the See also:

• FISH (O. Eng. fist, a word common to Teutonic languages, cf. Dutch visch, Ger. Fisch, Goth. fisks, cognate with the Lat. piscis)
• FISH, HAMILTON (1808-1893)

fish Notidanus, but the subject is discussed by T . J . See also:

• PARKER, JOHN HENRY (1806-1884)
• PARKER, JOSEPH (183o-1902)
• PARKER, MARTIN (c. 1600-c. 1656)
• PARKER, MATTHEW (1504-1575)
• PARKER, SAMUEL (164o-1688)
• PARKER, SIR GILBERT (1862— )
• PARKER, SIR HYDE, BART
• PARKER, THEODORE (1810-186o)

Parker and A .

M . Paterson (The Human Sternum, London, 1904, p . 50), and still requires further See also:

• RESEARCH (O. Fr. recerche, from recercher, re- and cercer, mod. chercher, to search; Late Lat. circare, to go round in a circle, to explore)

research . If the sternum be regarded as a segmental structure or series of segmental structures corresponding to the centra of the vertebrae there is no See also:

• REASON (Lat. ratio, through French raison)

reason why it should not develop independently of the intersegmental ribs and, when the ribs are suppressed, gain a secondary connexion with the shoulder girdle In Reptilia the centra of the vertebrae are usually procoelous, though there are a few examples, such as the archaic Tuatera See also:

• LIZARD (Lat. lacertal)

lizard (See also:

• SPHENODON, or TUATARA

Sphenodon), in which the amphicoelous arrangement persists . There are several cervical vertebrae instead of one, which is all the amphibians have . The odontoid bone is usually separate both from the atlas and axis while, between the atlas and the skull, there are rudiments of an extra intervertebral dorsal structure or pro-atlas in some forms such as the See also:

• CROCODILE

crocodile and Sphenodon lizard . Two sacral vertebrae (i.e. vertebrae articulating with the ilium) are generally See also:

• PRESENT

present instead of the one of the See also:

• AMPHIBIA

Amphibia, but they are not fused together as in mammals . In the tail region haemal arches are often found enclosing the caudal artery and vein as they are also in urodele amphibians; in some See also:

• SPECIES

species these are separate and are then spoken of as See also:

• CHEVRON (Fr. from chevre, a goat)

chevron bones . In the Crocodilia intervertebral disks first appear . Ribs are present in the cervical, thoracic and lumbar regions, and in the Chelonia (tortoises) the cervical ones blend with the vertebrae as they do in higher forms . In crocodiles a definite vertebrarterial canal is established in the cervical region which henceforward becomes permanent . The shafts of the ribs are sometimes all in one piece as in See also:

• SNAKES

snakes or they may be developed by three separate centres as in Sphenodon with intervening joints .

In these cases dorsal, intermediate and ventral elements to each shaft are present . In Crocodilia and Sphenodon there are spurs from each thoracic rib which overlap the next rib behind and are known as uncinate processes; they are developed in connexion with the origin of the See also:

• EXTERNAL

external oblique muscle of the abdomen and are very constant in birds . The ventral elements of some of the hinder ribs are found in the Croeodilia lying loose in the myocommata of the rectus and obliquus internus (inscriptiones tendineae) and are known as abdominal ribs, while the sacral vertebrae articulate with the ilium through the intervention of short rods of bone, sometimes called pleurapophyses, which are no doubt sacral ribs . The sternum of See also:

• REPTILES (Lat. Reptilia, creeping things, from reptilis; ref ere, to creep; Gr. Ep7rety, whence the term " herpetology," for the science dealing with them)

reptiles is a broad plate of cartilage which may be calcified but is seldom converted into true bone; it always articulates with the coracoids (see section Appendicular) anteriorly and with a variable number of ribs laterally and posteriorly . It should not be confounded with the See also:

• DAGGER

dagger-shaped interclavicle which, like the clavicles, is a membrane bone and overlaps the sternum ventrally . It is also probable that the interclavicle is morphologically quite distinct from the episternum, of which vestiges See also:

• ATE

ate present in man and are referred to above in the section on embryology (see fig . 27) . In birds the characteristics are largely reptilian with some specialized adaptations to their bipedal locomotion and See also:

• POWER [WILLIAM GRATTAN] TYRONE (1797-1841)

power of See also:

• FLIGHT

flight . One effect of this is that the two true sacral vertebrae become secondarily fused with the adjacent lumbar, caudal and even thoracic, and these again fuse with the ilium so that the posterior part of a See also:

• BIRD

bird's trunk is very rigid . The neck, on the other hand, Is very movable and the centra articulate by means of See also:

• SADDLE (a word common to Teutonic languages, cf. Ger. Sattel, Dut. zadel, also in Russ. siedlo and Lat. sella, for sedla; it is not derived directly from Lat. sedile, which means a chair, but all the words are to be referred to the root sad-, which gives

saddle-shaped joints which give the maximum of movement combined with strength (see JOINTS) . The caudal vertebrae are fused into a flattened bone, the pygostyle, to support the tail feathers . In the fossil bird See also:

• ARCHAEOPTERYX

Archaeopteryx the centra are amphicoelous and the long tail has separate caudal vertebrae .

The ribs are few and consist of dorsal (vertebral) and ventral (sternal) parts; the former almost always have uncinate processes . Free cervical ribs are often present and Archaeopteryx possessed abdominal ribs . The sternum is very large and in flying birds (Carinatae) has a median See also:

• KEEL

keel (carina) projecting from it, while the non-flying, See also:

• OSTRICH

ostrich-like birds (See also:

• RATITAE (from Lat. ratis, a raft)

Ratitae) have no such structure . In See also:

• MAMMALIA (from Lat. mamma, a teat or breast)

Mammalia the centra articulate by means of the intervertebral disks and it is only in this class that the epiphysial plates appear though these are absent in the See also:

• MONOTREMATA (a name referring to the single outlet for all the excretory channels of the body)

Monotremata (See also:

• DUCK

duck-See also:

• MOLE
• MOLE, LOUIS MATHIEU, COMTE (1781—1855)
• MOLE, MATHIEU (1584—1656)

mole, &c.) and Sirenia (See also:

• SEA (in O. Eng. sae, a common Teutonic word; cf. Ger. See, Dutch Zee, &c.; the ultimate source is uncertain)
• SEA, COMMAND OF THE

sea-cows) . The cervical vertebrae are with a few exceptions (two-toed and three-toed sloths and the manatee or sea-cow) always seven in number, and some, usually all, of them have a vertebrarterial canal in the transverse process . In some of the See also:

• CETACEA
• CETACEA (from the Gr. ,Eros, a whale)

Cetacea they are fused together . In the Ornithorhynchus the odontoid is a separate bone, as it is in many reptiles, but this part includes the facets by means of which the axis and atlas articulate . The thoracic 1 . For further details see C . S . McMurrich, The Development of the Human Body (London, 1906) . This includes bibliography, but G .

Ruge's See also:

• PAPER
• PAPER (Fr. papier, from Lat. papyrus)

paper on the development of the sternum (Morph . Jahrb. vi . 188o) is of See also:

• SPECIAL

special importance . Comparative Anatomy.—Just as in development the [notochord forms the earliest structure for stiffening the embryo, so in the animal See also:

• KINGDOM

kingdom it appears before the true backbone or vertebral column is evolved . This is so important that the older phylum of See also:

• VERTEBRATA

Vertebrata has now been See also:

• EXPANDED

expanded into that of Chordata to include all animals which either permanently or temporarily possess a notochord . In the subphylum Adelochorda, which includes the See also:

• WORM

worm-like See also:

• BALANOGLOSSUS

Balanoglossus, as well as the colonial forms Rhabdopleura and Cephalodiscus, an entodermal structure, apparently corresponding to the notochord of higher forms, is found in the dorsal wall of the pharynx . In the subphylum Urochorda or See also:

• TUNICATA

Tunicata, to which the ascidians or sea-squirts belong, the notochord is present in the tail region only and as a rule disappears after the See also:

• METAMORPHOSIS

metamorphosis from the larval to the adult form . In the See also:

• ACRANIA

Acrania, which are represented by See also:

• AMPHIOXUS, or LANCELET

Amphioxus (the lancelet). and are sometimes classed as the lowest See also:

• DIVISION (from Lat. dividere, to break up into parts, separate)

division of the subphylum Vertebrata, the notochord is permanent and extends the whole length of the animal . Both this and the nerve cord dorsal to it are enclosed in tubes of mesodermal connective tissue which are continuous with the fibrous myocom• mata between the myotomes . Here then is a notochord and a membranous vertebral column resembling a stage in man's development . In the See also:

• CYCLOSTOMATA, or MARSIPOBRANCHII

Cyclostomata (hags and lampreys) the notochord and` its sheath persist through life, but in the adult See also:

• LAMPREY

lamprey (Petromyzon) cartilaginous neural arches are developed . In cartilaginous ganoid fishes like the See also:

• STURGEON (Acipenser)

sturgeon, the notochord is persistent and has a strong fibrous sheath into which the cartilage from the neural arches encroaches while in the elasmobranch fishes (sharks and rays) the cartilaginous centra are formed and grow into the notochord, thus causing its partial absorption .

The growth is more marked peripherally than centrally, and so each centrum when removed is seen to be deeply concave toward both the head and tail; such a vertebra is spoken of as amphicoelous and with one exception is always found in fishes which have centra . In the body fish (Teleostei) and mud-fish (Dipnoi) the vertebrae are ossified . If a vertebra from the tail of a bony fish like the See also:

• HERRING (Clupea harengus, Haring in German, le hareng in French, sill in Swedish)

herring be examined, it will be seen to have a ventral (haemal) arch surrounding the caudal See also:

• BLOOD

blood-vessels and corresponding to the dorsal or neural arch which is also present . In the anterior or visceral part of the body the haemal arch is split and its two sides spread out deep to the muscles and lying between them and the coelom to form the ribs . In the elasmobranchs on the other hand the ribs lie among the muscles as they do in higher vertebrates, and the fact that both kinds of ribs are coexistent in the same segments in the interesting and archaic Nilotic fish Polypterus bichir shows that they are developed independently of one another . The sternum is never found in fishes with the possible exception of the See also:

• COMB (a word common in various forms to Teut. languages, cf. Ger. Kamm, the Indo-Europ. origin of which is seen in yoµros, a peg or pin, and Sanskrit, gambhas, a tooth)

comb-toothed See also:

• SHARK

shark (Notidanus) . Among the Amphibia the tailed forms (Urodela) have amphicoelous vertebrae in embryonic life and so have some of the vertebrae vary from ten in some of the whales and the peba See also:

• ARMADILLO

armadillo to twenty-four in the two-toed sloth, though thirteen or fourteen is the commonest number . In the anterior part of the thoracic region the spines point backward, while in the posterior thoracic and lumbar regions they have a forward direction . There is always one spine in the posterior thoracic region, which is vertical, and the vertebra which bears this is known as the anticlinal vertebra . The s Spinous process . m Metapophysis . az Anterior zygapophysis. pz Posterior zygapophysis. a Anapophysis .

t Transverse (costal) process . lumbar vertebrae vary freak two in the Ornithorhynchus and some of the armadillos to twenty-one in the See also:

• DOLPHIN

dolphin, the average number being probably six . Both the mammillary and accessory tubercles (See also:

• META

meta- and See also:

• ANA

ana-pophyses) are in some forms greatly enlarged . It is usually held that the former are morphologically muscular processes while the latter represent the transverse processes of the thoracic vertebrae . In the See also:

• AMERICAN

American edentates additional articular processes (zygapophyses) are developed, so that these animals are sometimes divided from the old-See also:

• WORLD

world edentates and spoken of as Xenarthra . Lying ventral to the intervertebral disks in many mammals small paired ossicles are occasionally found; these are called inter- centra and are ossifications in the hypochordal bar (see subsection on embryology) . They probably represent the places where the chevron bones or haemal arches would be attached and are the serial homologues of the anterior arch of the atlas (see fig. to) . Boulenger has pointed out that these intercentra, either as paired or median ossicles, are often found in lizards (P.Z.S., 1891, p . 114) . The sacrum See also:

• CON

con- sists of true sacral vertebrae, which directly articulate with the sacrum, and false, which are caudal vertebrae fused with the others to form a single bone . There is also reason to believe that vertebrae which are originally lumbar become secondarily included in the sacrum because in the develop- ment of man the pelvis is at first attached to the thirtieth vertebra, but gradually shifts forward until it reaches the twenty-fifth, twenty-sixth and twenty-seventh; the twenty- fifth or first sacral vertebra has, however, a frequent tendency to revert to the lumbar type and sometimes may do so on one side but not on the other . A .

Paterson, on the other hand, brings forward See also:

• EVIDENCE (Lat. evidentia, evideri, to appear clearly)

evidence to prove that the human sacrum undergoes a backward rather than a forward shifting (Scientif . Trans . R . Dublin Society, vol. v., ser . 11, p . I23) . Taking the vertebrae which fuse together as an arbitrary See also:

• DEFINITION (Lat. definitio, from de-finire, to set limits to, describe)

definition of the sacrum, we find that the number may vary from one in Cercopithecus parts to thirteen in some of the armadillos, and, if the Cetacea are included, seventeen in the See also:

• BOTTLE (Fr. bouteille, from a diminutive of the Lat. butta, a flask; cf. Eng. " butt ")

bottle-nosed dolphin, Tursiops . Four seems to be about the average of sacral vertebrae in the mammalian class and of these one or two are true sacral . In some of the See also:

• EDENTATA

Edentata the posterior sacral vertebrae are fused with the ischium, in other words the great sacro-sciatic ligament is ossified . The lateral centres of ossification which form the articular surface for the ilium probably represent rib elements . The caudal or tail vertebrae vary from none at all in the See also:

• BAT

bat Megaderma to See also:

• FORTY

forty-nine in the See also:

• PANGOLIN

pangolin (Manis macrura) . The anterior ones are remarkable for usually having chevron bones (shaped like a V) on the ventral surface of the intercentral articulation .

These protect the caudal vessels and give attachment to the ventral tail muscles . The ribs in mammals correspond in number to the thoracic vertebrae . In monotremes the three parts of the rib (dorsal, intermediate and ventral) already noticed in the reptiles are found, but usually the intermediate part is sup-pressed . The ventral part generally remains cartilaginous as it does in man though sometimes it ossifies as in the armadillos . In the typical pronograde mammals the shape of the ribs differs from that of the higher See also:

• PRIMATES
• PRIMATES (Lat. primas, first)

Primates and man : they are so curved that the dorso-ventral diameter of the thorax is greater than the transverse while in the higher Primates the thorax is broader from side to side than it is dorso-ventrally . In this respect the bats agree with man and the lemurs with the pronograde mammals . In some whales the first rib articulates by an apparently double head with two verte- brae; this is probably the result of a cervical• rib joining it a little way from the vertebral column, and the result is homologous with those cases in man in which a cervical rib joins the first thoracic as it sometimes does . In the toothed whales, of which the See also:

• PORPOISE (sometimes spelled Porpus and Porpesse)

porpoise is an example, the more posterior ribs lose their heads and necks and only articulate with the transverse processes . The sternum of mammals typically consists of from seven to nine narrow segments or sternebrae, the first of which (presternum) is often broader than those behind . As a rule the second rib articulates with the See also:

• INTERVAL

interval between the first and second pieces, but sometimes, as in the See also:

• GIBBON
• GIBBON, EDWARD (1737–1794)

gibbon, it is the third rib which does so . When this is the case, as it sometimes is in man, the first two sternebrae have probably fused (see A . See also:

• KEITH
• KEITH, FRANCIS EDWARD JAMES (1696-1758)
• KEITH, GEORGE (c. 1639-1716)