Online Encyclopedia

EXTERNAL

Online Encyclopedia
Originally appearing in Volume V19, Page 401 of the 1911 Encyclopedia Britannica.
Spread the word: del.icio.us del.icio.us it!
EXTERNAL BRAAt7/ second, third and fourth cervical nerves to form the cervical plexus, from which the skin of the side of the neck and lower part of the head and face are supplied by means of the small occipital, great auricular, superficial cervical, suprasternal, supraclavicular and supraacromial nerves (see fig. 7), as well as those muscles of the neck which are not supplied by the cranial nerves. The phrenic nerve, which comes chiefly from the fourth cervical, deserves special notice because it runs down, through the thorax, to supply the greater part of the diaphragm. The explanation of this long course (see DIAPHRAGM) is that the diaphragm is formed in the neck region of the embryo. The posterior primary division of the second cervical nerve is very large, and its inner (mesial) branch is called the great occipital and supplies most of the back of the scalp (fig. 7). The fifth, sixth, seventh and eighth anterior primary divisions of the cervical nerves as well as a large part of that of the first thoracic are prolonged into the arm, and in the lower part of the neck and armpit communicate with one another to form the brachial plexus. As a general law underlies the composition of the limb plexuses it will be worth while to study the structure and distribution of this one with some little care. It will be seen from the accompanying diagram (fig. 8) that each component nerve with the exception of the 'first thoracic divides into an anterior (ventral) and a posterior (dorsal) division which are best spoken of as secondary divisions in order to prevent any confusion with the anterior and posterior primary divisions which all the spinal nerves undergo. In the diagram the anterior secondary divisions are white, while the posterior are shaded. It has been suggested by A. M. Paterson that the posterior secondary branches correspond with the lateral branches of the thoracic nerves already mentioned, but there are still certain difficulties to be explained before altogether accepting this. Later on in the plexus three cords are formed of which the posterior is altogether made up of the posterior secondary divisions, while the anterior secondary divisions of the fifth, sixth and seventh cervical nerves form the c.w,n..rTM D. (mesial) branch through which the skin and muscles of the back are supply no skin. Its anterior primary division joins those of the supplied. It will be seen from the foregoing that the thoracic nerves are almost completely segmental in their distribution, in other words, From A. M. Paterson, in Cunningham's Text-Book of Anatomy. G.A., Great auricular nerve. 1.11, Intercostohumeral. S.C, Superficial cervical nerve. I.C, Internal cutaneous. S.C1, Supraclavicular nerves. M.S, Cutaneous branch of mus- Acr, Acromial. culo-spiral nerve. Cl, Clavicular. E.C, External cutaneous nerve. St, Sternal. G.C, Genito-crural nerve. T. 2-12, Lateral and anterior M.C1•2., Middle cutaneous nerve. branchesof thoracic nerves. LC', Branch of internal cutane- I.H, Ilio-hypogastric nerve. ous nerve. 1.1, Ilio-inguinal nerve. P, Branches of pudic nerve. Circ, Cutaneous branch of cir- S.Sc, Branches of small sciatic cumflex nerve. [nerve. nerve. L.I.C, Lesser internal cutaneous each supplies a slice of the body, but in the other regions this segmental character is masked by the development of the branchial skeleton and the limbs. In the cervical region the first cervical or suboccipital nerve comes out between the occiput and atlas and does not always have a posterior root. When it has not, it obviously can V sj s `~ §~ r •: From Gray's Anatomy, Descriptive and Surgical. external cord, and those of the eighth cervical and first thoracic the inner. As a general rule the nerves which rise from the ventral secondary divisions of the limb plexuses run only to that surface of the limb which was ventral in the embryo, while the dorsal secondary divisions are confined to the original dorsal area, but, in order to apply this to the human adult, it must be realized that the limbs outer head. of the median nerve (C. 5?, 6, 7), which joins the inner head (C. 8, Th. I) and supplies most of the flexor muscles of the front of the forearm as well as those of the ball of the thumb, the outer two lumbricals and also the skin of the outer part of the palm including the outer three digits and half the fourth. From the inner cord come the inner head of the median just mentioned, the ulnas nerve (C. 8, Th. I), which passes down behind the internal condyle of the humerus, where it is popularly known as the " tunny bone " and supplies the flexor carpi ulnaris, half the flexor profundus digitorum, and most of the muscles of the hand as well as the inner digit and a half on the palmar and dorsal aspects. Other branches of the inner cord are the internal cutaneous (C. 8, Th. I) supplying the inner side of the forearm, the lesser internal cutaneous (Th. I) which often joins the intercosto-humeral or lateral cutaneous branch of the second intercostal nerve to supply the skin on the inner side of the upper arm, and the internal anterior thoracic nerve (C. 8, Th. I) to the pectoralis minor and major. From the posterior cord are derived the three subscapular nerves (C. 5, 6, 7, 8) which supply the subscapularis, teres major and latissimus dorsi muscles, the circumflex nerve (C. 5, 6) supplying the deltoid and teres minor muscles, and the skin over the lower part of the deltoid, and the musculo-spiral nerve (C. 5, 6, 7, 8) which is the largest branch of the brachial plexus and gives off cutaneous twigs to the outer side and back of the arm and to the back of the forearm, as well as muscular twigs to the triceps and adjacent muscles.. At the elbow this nerve divides into the radial and posterior interosseous. The radial is entirely sensory and supplies the skin of the outer side of the back of the hand, including three digits and a half, while the posterior inter-osseous is wholly muscular, supplying the muscles on the back of the forearm. It will be seen that the posterior cord is derived altogether from posterior secondary divisions of the plexus, but there are three other nerves derived from these which should be mentioned. The posterior thoracic or respiratory Hypoglossal nerve of Bell comes off the back of the nerve fifth, sixth and seventh cervical nerves Internal laryn- before the anterior and posterior secondary Beal nerve divisions separate,and runs down to supply Nerve to the serratus magnus muscle. thyro-hyoid The posterior scapular or nerve to the Descendens rhomboid muscles runs to those muscles hypoglossi from the fifth cervical. The suprascapular nerve (C, 5, 6) passes through the soprasca.pular notch to supply the supraspinatus and infraspinatus muscles. The spinal nerves which are distributed to the lower limbs first intercommunicate in the lumbar and sacral plexuses, which, with the perineal nerves, are sometimes spoken of together as the lumbo sacral plexus. The lumbar plexus (see fig. 9) is formed as a rule of the first four lumbar nerves, though the greater part of the first number is segmental in its distribution and resembles one of the thoracic nerves. From A. M. Paterson, in Cunningham's Text-book of Anatomy. It early divides into an ilio-hypogastric wall in n the the run sub- by an overlapping region; and, secondly, that the area supplied by stance of the muscles, and of which the former gives off an iliac any one spinal nerve is liable to variation in different individuals within moderate limits. This variation may affect the whole plexus, and the term " prefixed plexus " has been devised by C. S. Sherrington to indicate one in which the spinal nerves entering into its formation are rather higher than usual, while, when the opposite is the case, the plexus is spoken of as " postfixed." With regard to the muscular supply of a limb the general rule is that each muscle is supplied by fibres derived from more than one spinal nerve; this, of course, is made possible by the redistribution of fibres in the plexuses. Moreover, the muscular supply does not necessarily correspond to that of the overlying skin, because (see MUSCULAR SYSTEM) some of the primitive muscles have been sup-pressed, others have fused together, while others have shifted their position to a considerable distance. Bearing the foregoing facts in mind, the main distribution of the nerves of the brachial plexus may be surveyed, though the exact details must be sought in the human anatomy text-books. The outer cord of the plexus gives off the external anterior thoracic nerve (C. 5, 6, 7) to the pectoralis major, the musculo-cutaneous nerve (C. 5, 6) to the muscles on the front of the arm, and to the skin of the outer side of the forearm and the are at one time flattened buds coming off at right angles from the side of the body and having dorsal and ventral surfaces, one (pre-axial) border toward the head of the embryo, and one (postaxial) toward the tail. If a person lies prone upon the floor with the arms outstretched and the palms downward the embryological position of the forelimb is to some extent restored, and it will now be easily understood that the more preaxial part of the limb will be supplied by those nerves which enter it from nearer the head, while the postaxial part draws its nerve supply from lower down the spinal cord. To use Herringham's words: " (A) Of two spots on the skin, that nearer the preaxial border tends to be supplied by the higher nerve. (B) Of two spots in the preaxial area the lower tends to be supplied by the lower nerve, and of two spots in the postaxial area the lower tends to be supplied by the higher nerve." Other points of general importance in regard to cutaneous nerve supply are, firstly, that the area of skin supplied by one spinal nerve is not sharply marked off from that of the next, but the two are separated Nerve to trapezius Acromial branches Clavicular fl' cervical Sternal plexus branch, which is in series with the lateral cutaneous branches of the intercostal nerves and passes over the crest of the ilium to the gluteal region, while the hypogastric branch runs round to the skin of the pubic region. The ilio-inguinal, on the other hand, gives off no lateral cutaneous or iliac branch, but is prolonged down the inguinal canal to supply the skin of the scrotum as well as that of the thigh which touches it. In all probability the hypogastric branch of the ilio-hypogastric and the whole of the ilio-inguinal represent the anterior secondary division of the first lumbar nerve, while the posterior secondary division is the iliac branch of the iliohypogastric. The other anterior secondary divisions of the lumbar plexus is the obturator (see fig. 8). The obturator nerve (L. 2, 3, 4) supplies the adductor group of muscles on the inner side of the thigh as well as the hip and knee joints; it occasionally has a cutaneous branch on the inner side of the thigh. The posterior secondary branches of the plexus are the genito-crural, the external cutaneous and the anterior crural. The genito-crural nerve (L. I, 2) is partly anterior (ventral) and partly posterior (dorsal). It sends one anterior branch through the inguinal canal to supply the cremaster muscle, and another (posterior) to the skin of the thigh just below cutaneous nerve supplies the peroneus longue and brevis muscles, the groin. The external cutaneous nerve (L.2, 3) supplies the skin of the outer side of the thigh, while the anterior crural (L.2, 3, 4) innervates the muscles on the front of the thigh, the skin on the front and inner From A. M. Paterson, in Cunningham's Text-Book of Anatomy. Sy, Sympathetic gangliated cord. Cb, Nerve to coraco-brachialis. Phr, Phrenic nerve. M, Median nerve. C.4, 5, 6, 7, 8, T.I, 2, 3, Anterior primary divi- Inner Cord. sions of the lower cervical and upper I A.T, Internal anterior thoracic nerve. thoracic nerves. U, Ulnae nerve. ntl, mz, Muscular branches to axial muscles. LC, Internal cutaneous nerve. P.T, Long thoracic nerve. L.I C Lesser internal cutaneous nerve. Rh, Nerve to rhomboids (posterior scapular). Subcl, Nerve to subclavius muscle. Posterior Cord. Int, Intercostal nerves. Circ, Circumflex nerve. S.Sc, Supra-scapular nerve. The intercostal M.S, Musculo-spiral nerve. part of the first thoracic nerve is omitted. S.Sub, Short subscapular nerve. Outer Cord. M.Sub, Lower subscapular nerve. L.Sub, Long subscapular nerve. F_.A.T, External anterior thoracic nerve. Intercosto-humeral nerve. perineum, buttock and the back of the M .C, Muscular-cutaneous nerve. Let, Lateral branch of third intercostal nerve. thigh. The pudic nerve (S.2, 3, 4) helps side of the thigh, through its middle and internal cutaneous branches, to supply the skin and muscles of the perineum and genital and the skin of the inner side of the leg and foot through the internal organs. The visceral branches form the pelvic stream of white saphenous branch. At first sight it is difficult to understand how the rami communicantes (see NERVOUS SYSTEM) ; they run from anterior crural nerve, which supplies the skin of the front of the the second and third or third and fourth sacral nerves to the pelvic thigh, is a posterior secondary division of the lumbar plexus, but plexuses of the sympathetic system. The perforating cutaneous the explanation is that the front of the human thigh was originally nerve (S.2, 3) pierces the great sacro-sciatic ligament and supplies the dorsal surface of the limb bud, and the distribution of the nerve the skin over the lower internal part of the buttock. The muscular is quite easily understood if the position of the hind limb of a lizard branches (S.3, 4) supply the external sphincter, levator ani and or crocodile is glanced at. The fourth lumbar nerve is sometimes coccygeus. called the nervus furcalis, because, dividing, it partly goes to the The sacro-coccygeal nerve (S.4, 5, Coc.i) runs down on each side lumbar, and partly to the sacral plexus (fig. 8), though, when the of the coccyx to supply the adjacent skin, and representstheventro- plexusis prefixed, the third lumbar may be the nervus furcalis, or, lateral nerve of the tail of lower mammals. (F. G. P.) when it is postfixed, the fifth lumbar. Under ordinary conditions the descending branch of the fourth lumbar nerve joins the fifth, and NERVI, a coast town of Liguria, Italy, in the province of Genoa, together they make the lumbo-sacral cord, which, with the first three from which it is 72 M. S.E. by rail (also electric tramway), 82 ft. sacral nerves, forms the sacral plexus. This plexus, like the others, above sea-level. Pop. (1901) 3480 (town); 63x7 ,(commune). contains anterior and posterior secondary divisions of its spinal It is much frequented as a winter resort. It is surrounded with nerves, and it resembles the brachial plexus in that the lowest nerve to enter it contributes no dorsal secondary division. groves of olives, oranges and lemons, and its villas have beautiful All the constituent nerves of the plexus run into one huge nerve, gardens. It is moister and less dusty than the western Riviera, the great sciatic, which runs down the back of the thigh and, before and is especially in favour with those who suffer from lung reaching the knee, divides into external and internal popliteal nerves. 1 These two nerves are sometimes separate from their first formation , complaints. At Quarto, 21 M. N.W., I00o Garibaldians (r Mille) in the plexus, and may always be separated easily by the handle of embarked for Marsala in 186o. a scalpel, since they are only bound together by loose connective NERVOUS SYSTEM. The nervous system, forms an extremely tissue to form the great sciatic nerve. When they are separated in complicated set of links between different parts of the body, this way it is seen that the external popliteal is made up entirely and is divided into A the central nervous system, composed of of posterior (dorsal) secondary divisions (see fig. 9), and is derived ( ) from the fourth and fifth lumbar and first and second sacral nerves, (I) the brain, and (2) spinal cord; (B) the peripheral nervous while the internal popliteal is formed by the anterior (ventral) system, consisting of (I) the cranial nerves, (2) the spinal nerves, secondary divisions of the fourth and fifth lumbar and first, second (3) the various sense organs, such as the eye, ear, olfactory organ, and third- sacral nerves. The external popliteal nerve supplies the short head of the biceps femoris (see MUSCULAR SYSTEM), and, just taste organ and tactile organs, and (4) the motor end plates; below the knee, divides into anterior tibial and musculo-cutaneous (C) the sympathetic system. The anatomy and physiology of branches, which both supply the dorsal surface of the leg and foot. many of these parts are treated in separate articles (see BRAIN, The anterior tibial nerve is chiefly muscular, innervating the muscles SPINAL Co, RDNERVE, EYE, EAR, OLFACTORY cutaneous TOUCH, MUSCLE AND NERVE, SYMPATHETIC NERVOUS SYSTEM). branch to the cleft between the first and second toes. The musculo- The object here is to deal with anatomical points which are and the rest of the skin of the dorsum, of the foot, and lower part of the leg, while the skin of the upper part of the. dorsum of the leg, below the knee, is supplied by the external popliteal before its division. The internal popliteal nerve, after supplying the ham-strings, is continued into the calf of the leg as the posterior tibial and innervates all the muscles on this, the ventral, surface. Behind the inner ankle it divides into the external and internal plantar nerves, from which the muscles and skin of the sole are supplied. A little above the knee each popliteal nerve gives off' a contribution to help form the external or short saphenous nerve. That from the internal popliteal is called the communicans tibialis, while that from the • external popliteal is the communicans fibularis. These join about the middle of the back of the calf, and the, now formed, short saphenous nerve 'runs down behind the outer ankle to , supply the outer side of the foot. Some-times it encroaches on the dorsum of the foot, replacing part of the musculo-cutaneous, though, when this is the case, its dorsal contribution from the external popliteal (communicans fibularis) is always larger than usual. To return to the sacral plexus: branches are given off from the anterior secondary divisions to the short external rotator muscles of the hip (pyriformis, quadratus femoris, &c.), while from the posterior secondary divisions come the superior gluteal (L. I.S. 4, 5) and the inferior gluteal (L.5, S. I, 2) to the muscles of the buttocks. In modern descriptions the lower branches of the lumbo-sacral plexus are grouped into a pudendal plexus, and the plan, though open to criticism on morphological grounds, has such descriptive advantages that it is followed here. Contributions from the first, second, third and fourth sacral, and the coccygeal nerve, form it, and these contributions are almost all anterior (ventral) secondary divisions. The branches of this plexus are the small sciatic, pudic, visceral, perforating cutaneous, muscular and sacro-coccygeal nerves. The small sciatic (S.I, 2, 3) is partly dorsal and partly ventral in its origin and distribution; it supplies the skin of the conveniently occur elsewhere.
End of Article: EXTERNAL
[back]
EXTERIOR POLICY
[next]
EXTERRITORIALITY

Additional information and Comments

There are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click and select "copy." Paste it into a website, email, or other HTML document.