Nerve

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) sup plying 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 alto gether 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 supra scapular notch to supply the supraspinatus and infraspinatus muscles.

Lumbo-Sacral Plexus.

The spinal nerves which are dis tributed to the lower limbs first intercommunicate in the lum bar and sacral plexuses, which, with the perineal nerves, are some times spoken of together as the lumbo-sacral plexus. The lumbar plexus (fig. 9) is formed as a rule of the first four lumbar nerves, though the greater part of the first lumbar 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 derived from more than one spinal nerve; this is made possible by the redistribution of fibres in the plexuses. Moreover, the mus cular supply does not necessarily correspond to that of the over lying skin, because (see MUSCULAR SYSTEM) some of the primi tive 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. 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. i) and sup

plies most of the flexor muscles of the front of the forearm as well as those of the ball of the thumb.

From the inner cord come the inner head of the median just mentioned, the ulnar nerve (C. 8, Th. I), 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. i) supplying the inner side of the forearm, a av• nirtnytt. rit rytLra. brivwiriu nt Nt.KVLb 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 other anterior secondary division of the lumbar plexus is the obturator (fig. 9). 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 cuta neous and the anterior crural. The genito-crural nerve (L. r, 2) is partly anterior (ventral) and partly posterior (dorsal). It sends one anterior branch through the inguinal canal to supply the cre master muscle, and another (posterior) to the skin of the thigh just below 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) inner vates the muscles on the front of the thigh, the skin on the front and inner side of the thigh, through its middle and internal cuta neous branches, and the skin of the inner side of the leg and foot through the internal saphenous branch. At first sight it is difficult to understand how the anterior crural nerve, which supplies the skin of the front of the thigh, is a posterior secondary division of the lumbar plexus, but the explanation is that the front of the human thigh was originally the dorsal surface of the limb bud, and the distribution of the nerve is quite easily understood if the position of the hind limb of a lizard or crocodile is glanced at. The fourth lumbar nerve is sometimes called the nervus furcalis, because, dividing, it partly goes to the lumbar, and partly to the sacral plexus (fig. 9), though, when the plexus is prefixed, the third lumbar may be the nervus furcalis, or, when it is postfixed, the fifth lumbar.