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Originally appearing in Volume V25, Page 200 of the 1911 Encyclopedia Britannica.
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THE SKULL IN SAGITTAL SECTION. If the skull be sawn down just to the right of the mid line and the left half be looked at, the appearance will be that reproduced in fig. 6. The section of the cranial bones shows that they are formed of an outer and inner table of hard bone, while between the two is a layer of cancellous tissue called the diploe. In certain places the diploe is invaded by ingrowths from the air passage which separate the two tables and form the air sinuses of the skull, though it is important not to confuse these with the intracranial blood or venous sinuses. In the section under consideration two of these spaces, the frontal (f s) and the sphenoidal (PS) air sinuses are seen. ; Behind the frontal sinus is the crista Mesial Plane (see also lettering in fig. 2) : BO, Basi-occipital. SC, Septal cartilage of nose. V, Vomer. P1, Pt, s Pf, fm, a, s, galli already mentioned, while below is the bony septum of the nose formed, by the mes-ethmoid plate (ME), the vomer (V), and the line of junction of the palatine processes of the two maxillae and two palate bones. The re-entering angle between the mes-ethmoid and vomer is filled in the recent state by the septal cartilage (SC). Below the face is the inner surface of the body and ramus of the mandible, and half-way down the latter is the inferior dental foramen where the inferior dental branch of the fifth nerve accompanied by its. artery passes into the inferior dental canal in the substance of the bone to supply the lower teeth. Just in front of this foramen is a little tongue of bone called the lingula attaching the spheno-mandibular (long internal lateral) ligament, while running downward and forward from this is the mylo-hyoid ridge with' the groove of the same name just below it. If the cut surface of the right half of the skull be looked at, the outer wall of the nasal cavity will be seen with the three turbinated bones each overhanging its own meatus, but the anatomy of this part has already been dealt with in the article on the olfactory system (q.v.). For further details see any standard anatomical textbook—Quain, Gray, Cunningham, &c. For charm of style, The Human Skeleton by G. M. Humphry (London, 1858), although somewhat out of date, is unsurpassed. Embryology. The notochord (see SKELETON: Axial) extends forward to the ventral surface of the middle cerebral vesicle (see BRAIN) or as far EO, Ex-occipital. PT, Petrous-temporal. BS, Basi-sphenoid. PS, Pre-sphenoid (the letters are placed in the sphenoidal sinus). OS, Orbito-sphenoid. ME, Mes-ethmoid. Palate. Pterygoid of sphenoid. Frontal sinus. Pituitary fossa. Foramen magnum. Angle. Symphysis of lower jaw. as the place where the dorsum sellae will be. It is partly surrounded by the mesenchyme just as it is completely in the rest of the axial skeleton, and this mesenchyme extends dorsally on each side to wrap round the nerve cord, which is here the brain. In this way the brain becomes enclosed in a primitive membranous cranium, the inner part of which persists in its primitive condition as the dura mater, while the outer part may chondrity, chondrify and ossify, or ossify without a cartilage stage. That part of the cranium which is in front of the notochord is called prechordal, while the posterior part into which the notochord extends is chordal. On each side of the notochord chondrification takes place and a basicranial plate of cartilage is formed which soon meets its fellow of thwpposite side; and forms the floor of the skull as far forward as the dorsum sellae, and as far back as the external occipital protuberance. Laterally it comes in contact with the mesenchyme surrounding the internal ear, which is also chondrifying to form the cartilaginous periotic capsule, and the two structures fuse together to form a continuous floor for the back of the skull. A. Froriep has shown that in the hinder occipital region of the calf there are evidences of four vertebrae having been incorporated with the basicranial plate, that is to say that the plate and its coalesced vertebrae represent five mesodermic somites (" Zur Entwickelungsgeschichte der Wirbelsaule, insbesondere des Atlas and Epistropheus und der Occipitalregion," Archie fur Anat. u. Phys., Anat. Abth., 1886). It has more recently been shown by Levi that the same thing is true for man. K. Gegenbauer has pointed out that the primitive membranous skull shows, in the chordal region, signs of metameric segmentation in the way in which the cranial nerves pierce the dura mater one behind the other. These segments, however, had lost their distinctness even before the cartilaginous cranium had become developed, so that there is no real segmental value in the elements of this, still less in those of the bony skull. The only place in which segmental elements can be distinguished is in the occipital region, which is in structure transitional between the head and vertebral column. The notochord, it has been shown, ends just behind the place where the stomodaeum pouches up through the cranial base to form the anterior part of the pituitary body (see BRAIN). Where it ends two curved bars of cartilage are formed, which run forward till they meet the olfactory capsules, which are also now chondrifying. These bars are the prechordal cartilages or trabeculae cranil and enclose between them the crania-pharyngeal canal by which the pituitary body ascends, but later on, as they grow, they join together and cut off the pituitary body from the pharynx. By their growth outward they form the floor of the prechordal part of the chondro-cranium, so that from them is developed that part of the cartilaginous skull which will later on be part of the basisphenoid, the presphenoid, orbitosphenoid and alisphenoid regions. It has hitherto been assumed that this process held good for man, but recent research shows that the anterior part of the base of the skull chondrifies in the same way that ice appears on a pond and that the trabeculae are at no time definite structures. Chondrification of the nasal capsules is later than that of the parts of the skull behind, so that there is a steady progress in the process from the occipital to the ethmoidal region. There is a median centre of chondrification, the mesethmoid cartilage, which projects down into the fronto-nasal process (see OLFACTORY SYSTEM), and two lateral ectethmoid cartilages which eventually join with the mesethmoid to form the cartilaginous ethmoid. The cartilaginous base of the cranium is now formed, but the vault is membranous. While the base has been developing the two anterior visceral arches have been also forming and have gained an attachment to the cranium, but the formation and fate of these is recorded in the article SKELETON (Visceral). About the sixth week of foetal life ossification begins at different points in the membranous vault of the skull. In this way the frontal, parietal, supra-occipital, c b a b c and a little later the squamous part of the temporal bones are formed. About the eighth week, too, the lachrymal, nasal and vomer appear in the membrane lying superficial to different parts of the olfactory capsule. All these are dermal bones, comparable to the deeper parts of the scales of fishes, and developed in the mesenchyme lying deep to and in con-tact with the ectoderm. It is therefore necessary to think of the primitive skull as a three-layered structure, the deepest layer persisting as the dura mater, the middle forming the chondro-cranium, which ossifies to form the base, and a superficial layer close to the skin or mucous membrane (ectoderm), from which the bones of the vault and superficial parts of the olfactorycapsules are derived. At the four angles of the parietal, ossification is checked for some time to form fontanelles, of which the bregma is the most important, and at each of these points, as well as elsewhere in the sutures, accessory centres of ossification may g occur to form Wormian bones. Along the middle line of the base of the skull the same progress of ossification from behind forward is seen that was noticed in f the process of chondrification. Bilateral centres for the basioccipital appear about the sixth week, for the basisphenoid in the eighth, and for the pre-sphenoid in the tenth, while the lateral mass of the ethmoid does not ossify till the fifth month and the mesethmoid not until the first year of extra-uterine life. In the lateral part of the base the ex-occipitals and alisphenoids begin to ossify about the eighth week and the presphenoids about the tenth. In connexion with the alisphenoid there is a small extra centre of morphological interest only, which forms a little tongue-shaped process called the lingula, projecting back into the middle lacerated foramen and apparently corresponding to the sphenotic bone of lower vertebrates. The auditory or periotic capsule, like the olfactory, is late in ossifying; it has four centres (pro-otic, epiotic, opisthotic and pterotic) which do not come until the fifth month. Some parts of the chondro-cranium do not ossify at all; this is the case in the anterior part of the mesethmoid, which remains as the septal cartilage of the nose, while, as has been already pointed out, a buffer of cartilage persists between the basioccipital and basisphenoid until the twentieth year of life. From what has been said it is evident, and it will be still more evident if the article SKELETON h (Visceral) be looked at, that some of the bones of the adult skull are compounded of various contributions from the different elements which make up the adult cranium. These, recapitulated, are (I) the dura mater or entocranium, which in man does not ossify except perhaps in the crista galli. (2) The chondro-cranium or mesocranium. (3) The superficial part of the mesenchyme (ectocranium) from which dermal bones are formed. (4) The olfactory and auditory sense capsules. (5) The visceral arches. (6) Some fused vertebrae posteriorly. The occipital bone, for ex-ample, has the basioccipital, exoccipital and basal part of the supra-occipital derived from the chondro-cranium and fused vertebrae, while the c Fenestra rotunda. vault part of the supra- d, Foramen ovate. occipital has four dermal e, Mastoid. centres of ossification corre- f' Mastoid process. spondina to the interparietal g, Masto-squamosal suture, with and preinterparietal bones of foramen for transmission of lower mammals (see fig. 4). In h, Sq vessel. uamso zygomatic. the accompanying figure the latter centres have fused with the interparietal, but an indication of their line of junction is seen on each side of g. The bone of Kerkring (c) is an abnormality, the meaning of which is not understood. The temporal is also a very composite bone; in it the petromastoid portion represents the auditory sense capsule; the tabular d e d Arthur Thomson, in Cunningham'; Text-Book of Anatomy. a Arthur Thomson, in Cunningham's Text-Book of Anatomy. a, Basilar centre. b, Exoccipital. c, Ossicle of Kerkring. d, Supra-occipital (from cartilage). e, Fissure between supra-occipital and interparietal. f, Interparietal (from membrane). g, Fissure between interparietals. b a Arthur Thomson, in Cunningham's Text-Book of Anatomy. a, Tympanic ring. b, Inner wall of tympanum. external auditory meatus is formed by the outgrowth of the tympanic ring (fig. 9, a) which is probably part of the first visceral arch (see SKELETON, Visceral) ; the squamozygomatic part is a dermal bone, while the styloid process is a part of the second visceral arch. The mastoid process is not present at birth, but appears about the second year and becomes pneumatic about puberty. From what has been seen of the skull bones in the above necessarily concentrated and abridged account, it is obvious that they do not correspond to the traces of segmentation as indicated by the cranial nerves, and for this and other reasons the " vertebrate theory of the skull " is no longer believed in. For further details and references see Quain's Anatomy (London, 1908) ; Cunningham's Anatomy (Edinburgh, 1906); The Development of the Human Body, J. P. McMurrich (London, 1906). Comparative Anatomy. In this section only those parts of the skull which form the covering for the brain and the capsules for the olfactory and auditory apparatus are considered. Those parts of the face and jaws which are developed in connexion with the visceral arches are dealt with in the article SKELETON (Visceral). In the Acrania (Amphioxus) the enlarged anterior end of the nerve cord is merely surrounded by fibrous tissue continuous with the sheath of the rest of the nerve cord; there is therefore, in a sense, no true cranium. In the Cyclostomata (hags and lampreys) a cartilaginous cranium is developed, the anterior part of which forms an unpaired olfactory capsule connected with the rest of the cranium by fibrous tissue only. In the floor, just in front of the anterior end of the notochord, an aperture, the basi-cranial fontanelle, remains unchondrified for the passage of the pituitary diverticulum into the skull. In the Elasmobranchit (sharks and rays) and Holocephali (Chimaera) among the fishes the skull is still a complete cartilaginous box, though calcification of the cartilage often takes place. Taking the skull of the dogfish as a type, two large olfactory capsules are seen in front, and behind these the cranial brain-box is narrowed, being excavated at its sides for the great orbits. More posteriorly the auditory capsules widen the skull, and on the posterior (caudal) aspect the foramen magnum is seen with an occipital condyle on each side of it for the first vertebra to articulate with. On the upper (dorsal) surface of the skull are two apertures in the middle line; the more anterior of these is sometimes called the anterior fontanelle, though it has nothing to do with the bregma, described in man's skull, but forms a rudimentary median orbit for the pineal eye (see BRAIN). The posterior fontanelle is a depression which leads into two lateral tubes, each of which passes into the auditory capsule and is known as an aqueductus vestibuli (see EAR). In the cartilaginous ganoid fishes (sturgeon), which, like the elasmobranchs, are of great antiquity, the chondro-cranium is partly ossified so that all- and orbito-sphenoids are found; in addition to this a large number of dermal bones have made their appearance, such as nasals, frontals, parietals, supra and post ternporals, while in the roof of the mouth and pharynx a long membrane bone, the parasphenoid, is formed, and lies ventral to and strengthens the cartilaginous base of the skull. It will be noticed that these fish are important morphological landmarks, because in them the almost unchanged chondro-cranium coexists with a dermal ectocranium. In the bony ganoids such as the " bow fin " (Amia) the dermal bones are still more numerous and, among others, squamosals, prootics and exoccipitals appear. These fish are also remarkable for a fusion of the anterior part of the vertebral column with the occipital region of the skull, an arrangement recalling Froriep's observations on the skull of the calf embryo mentioned in the section on embryology. In the bony fishes (Teleostei) the membrane or dermal bones are still more numerous, and many of them are unrepresented in the mammalian skull, while others, which are there quite rudimentary, are very large. The chondro-cranium tends to disappear in the vault, but the base is fully ossified. Among other cartilage bones the five ossifications of the auditory capsule are seen, the pro-, epi-, opisth-, pter- and sphen-otics, all of which are found as centres of ossification in man. In the cod, for example, the sphenotic, which is represented in man by the little lingula sphenoidahs, is larger than the alisphenoid. In the Dipnoi (mud-fish) the chondro-cranium is very slightly ossified, only exoccipitals being found, but there is the,same coalescence with anterior vertebrae which was noticed in the ganoids. Dermal bones are plentiful. In the Amphibia the chondro-cranium persists and is only ossified in front by the girdle bone or sphenethmoid, and behind by the prootics and exoccipitals, the latter of which bear the two condyles. The anterior fontanelle is well marked in the chondro-cranium, but is completely overlaid and concealed by the dermal fronto-parietals. The membrane bones though large are much less numerous than in the bony fishes. In the Reptilia the skull varies immensely in the different orders, but speaking broadly, the chondro-cranium is less distinct than in the Amphibia, except in the ethmoidal region. In the base of the skull the basioccipital and basisphenoid are tending to replace the membranous parasphenoid, and instead of two exoccipital condyles only one in the mid line is present, though this in many forms (e.g. Chelonia) consists of three parts, a median borne on the basioccipital and two lateral on the exoccipitals. The parietal foramen is usually definitely marked in the dermal part of the skull and forms a median orbit for the pineal eye; this is especially the case in the Lacertilia (lizards). Except in the Ophidia (snakes) and Amphisbaenidae (worm-like lizards) there is a fibro-cartilaginous septum between the orbits so that the cranial cavity does not reach forward to the ethmoidal region. The pro-, epi- and opisth-otic bones are all developed, but the epiotic usually fuses with the supra-occipital and the opisthotic with the exoccipital. In the Crocodilia the first attempt at pneumaticity is seen in the basisphenoid, which is traversed by a complicated system of Eustachian passages leading eventually to the tympanum. In the class Aves the general scheme of the reptilian skull is maintained, though the bones fuse together very early, thus obliterating the sutures between them. Almost all of them have air in their interior, and so are said to be pneumatic. The single occipital condyle, if looked at in a young specimen, is seen to consist of a basioccipital and two exoccipital elements, though these are indistinguishable in the adult. The parasphenoid is represented by a broad plate which is called the basitemporal. The pro-, epi- and opisth-otic bones fuse together to form the auditory capsule. In the Mammalia the calvaria varies considerably in the different orders, the characteristic features being best marked in adult males. Usually the different bones are interlocked by sutures, as in man, until adult life, but in some orders (e.g. Monotremata, Edentata and Carnivora) they f use together quite early. In the basicranium the cartilage bones presphenoid, basisphenoid, and basioccipital, are so well developed that the parasphenoid has disappeared. In the basisphenoid of the rabbit the cranio-pharyngeal canal (see section on embryology) persists as a foramen at the bottom of the pituitary fossa. In the lower orders the face lies well in front of the brain case, as it does in reptiles and amphibians, but as the Primates are reached the increasing size of the calvaria causes it to overlie the face. Many of the bones are pneumatic, the process reaching its maximum in the elephant and the adult male gorilla. The periotic capsule blends with the squamosal and tympanic to form the petrous bone, though it is practically only in man that the second visceral arch ossifies on to this as a styloid process. There are usually two occipital condyles which have basis and exoccipital elements, though there are many mammals in which there is one large crescentic condyle surrounding the anterior half of the foramen magnum. Ossification of the processes of the dura mater occurs in the tentorium cerebelli of the carnivora and in the falx cerebri of the ornithorhynchus and porpoise. The orbits are in most mammals continuous with the temporal fossae. Sometimes, as in many of the ungulates and in the lemurs, they are outlined by a bony ring, but it is not until the higher Primates are reached that the two cavities are shut off and even then a vestige of their original continuity remains in the spheno-maxillary fissure. For further details see W. H. Flower, Osteology of the Mammalia (London, 1885) ; S. H. Reynolds, The Vertebrate Skeleton (Cambridge, 1897); R. Wiedersheim; C. Gegenbaur, Vergleich. Anat. der Wirbelthiere (Leipzig, 1901). (F. G. P.)
THE SKULL FROM BEHIND (norma occipitalis) (fig. 3)

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