\ ERNE t l.a.i S IF THE C111111. in describing the origin of fibre systems of the cord. ninny of the «•Ils of the cold have been necessarily referred tn. e.g. those veils the axones of %vhieli form thee odium's of Dowers and of Plechsig, and the flindainental 4.4)1111iitts. There are also found in the gray matter cells wide]] have short axones whirl ne%er leave the• gray 'natter, but terminate in the vieinity of their cells of origin. The most important groups of cells remaining to be de the mixed spinal nerve. The fibres of the sensory root have been already described in connection with the posterior odium's. 'fliew are the cen tral processes of the spinal ganglion cells. The peripheral processes of these cells pass out of the outer end of the ganglion and join the motor fibres. The nerves leave the spinal canal through the interVertebral foramina. beyond which each nerve divides into two branches. a posterior and all anterior.
The posterior divisions of the spinal nerves are smaller than the anterior and are distributed to the muscles and skin of the back.
The suterior divisions of the spinal nerves supply the anterior parts of the body and the extremities. Each anterior branch is connected by some slender filaments with the sympathetic. In the dorsal region the anterior divisions are separate and distributed quite evenly, but in the cervical, lumbar, and sacral regions they unite to form intricate plexuses.
TnE CEnvicAL PLEXUS. This is formed by the anterior divisions of the first four cervical nerves. The branches of distribution of this plexus may be divided into a superficial set and a deep set, the former supplying the integument and the more superficial muscles of the side and front of the neck and upper part of the throat ; the latter supplying the more deeply seated muscles of the neck and sending one very important branch, the phrenic, to the diaphragm.
The brachial plexus is formed by the anterior divisions of the last four cervical and the first dorsal nerves. These five nerves unite to form three main cords known respectively as the outer, the inner, and the posterior cords of the brachial plexus. Before uniting to form these cords, four main branches or sets of branches are given off. These are (1) a communicating branch to the phrenie; (2) muscular branches to the longus eolli, scaleni, rhomboidei. and subelavius; (3) posterior thoracic nerve, to the serrates mag nus; (4) the supra-scapular, sending branches to supply the supra- and infra-spinatus. From
the three main cords of the plexus branches are given off to the chest, shoulders, and arms. Those to the chest are the external and internal ante rior thoracic, which supply the peetoralis major and pectoris minor muscles. The branches to the shoulder are the subscapular and the circum flex, the former supplying the subscapularis, the latissimus dorsi, and the teres major; the latter supplying the deltoid and the teres minor. The circumflex nerve also supplies part of the in tegument of the shoulder and sends filaments to the musculo-cutaneons nerve. This nerve in the arm supplies the biceps, coraco-brachialis. and part of the brachialis antieus muscles. In the forearm it becomes cutaneous, supplying the in tegument on the radial side. The internal cu taneous nerve is a small nerve to the integument of the front of the arm and forearm. The lesser internal cutaneous or nerve of Wrisberg, the smallest branch of the plexus, is a branch to the integument on the inner side of the arm. The musculo-spira I supplies the triceps, aneoneus, supinator longus, extensor ca r pi radialis longior, and part of the braeh ia is ant i ens. It sends a cutaneous branch to the skin of the outer part of the forearm. It then divides into radial and pos ter i o r interns seous branches, the former pass ing to the skin of the radial side of the hand, inelud ing thumb, index, middle and radial side of ring fin gers; the latter supplying the carpal articu lations and muscles of the back of the fore arm. The ulnar nerve supplies the flexor carpi ulnaris and inner part of the flexor pro. fundus digitorum. In the hand it supplies the adductor and part of the short flexor of the thumb, the interossci, the two inner lumbrieales, and the muscles of the ball of the little finger. It supplies a cutaneous branch to the back of the little and ulnar side of the ring fingers, and to the front of the same fingers. It also supplies articular branches to the elbow and wrist joints. The median nerve supplies all the flexors and pronators of the forearm except those supplied by the uluar. In the hand it supplies the two outer lumbrical muscles, part of the short flexor of the thumb, the opponens and the abductor. It also innervates the skin of the paten and of the palmar surfaces of the fingers not supplied by the ulnar.