are at one time flattened buds coming off at right angles from the side of the body and having dorsal and ventral surfaces, one (preaxial) 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 by an overlapping region; and, secondly, that the area supplied by 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.”
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| From A. M. Paterson, in Cunningham’s Text-book of Anatomy. |
| Fig 7.—The Triangles of the Neck (Nerves). |
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 suppressed, 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 outer head of the median nerve (C. 5?, 6, 7), which joins the inner head (C. 8, Th. 1) 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 ulnar nerve (C. 8, Th. 1), which passes down behind the internal condyle of the humerus, where it is popularly known as the “funny 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. 1) supplying the inner side of the forearm, the lesser internal cutaneous (Th. 1) 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. 1) 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 ma]or 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 interosseous 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 nerve of Bell comes off the back of the fifth, sixth and seventh cervical nerves before the anterior and posterior secondary divisions separate, and runs down to supply the serratus magnus muscle.
The posterior scapular or nerve to the rhomboid muscles runs to those muscles from the fifth cervical.
The suprascapular nerve (C, 5, 6) passes through the suprascapular 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. It early divides into an ilio-hypogastric and ilio-inguinal branch, which run round the abdominal wall in the substance of the muscles, and of which the former gives off an iliac 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 ilio-hypogastric.
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. 1, 2) is partly anterior (ventral) and partly posterior (dorsal). It sends one anterior branch through the inguinal canal to supply the cremaster
