868 ANATOMY [NEBVOUS the upper arm; lience they are called intercosto-humeral nerves. Nervous In the regions of the neck, loins, and pelvis, the anterior plexuses, divisions of the spinal nerves do not pass simply outwards to their distribution. In each region adjacent nerves in terlace with each other, and form what is technically called a nervous plexus. When a branch arises from a thoracic nerve, it contains fibres derived from that nerve only ; but when a branch arises from a plexus, it may contain^ fibres, not of one only, but of two or more of the nerves which, by their interlacement, form the plexus. Hence the parts which are supplied by these branches are brought into con nection with a greater number of nerves, and consequently with a greater extent of the spinal cord or nerve centre, than are the parts which receive branches from a single nerve only. These plexuses are especially found in con nection with the nerves which supply the extremities, where, owing to the complexity of the muscular move ments, the co-ordination of these movements through the nervous system is rendered necessary. The anterior divisions of the eight cervical nerves are arranged in two plexuses, named cervical and brachial. The Cervical plexus (PL XVII.) is formed of the four upper cervical nerves, which make, by interlacement with each other, a series of loops in front of the transverse processes of the cervical vertebrae. Arising either directly from these nerves, or from the plexus which they form, are communicating, muscular, and cutaneous branches. The Communicating branches connect these nerves with the large superior cervical ganglion of the sympathetic system, also with the vagus, accessory, and hypoglossal cranial nerves, and with the descending branch of the hypoglossal. The Muscular branches supply the anterior recti muscles of the neck, the levator scapulae, the posterior scalenus, the diaphragm, and in part the sterno-rnastoid and trape- zius. The branch to the diaphragm, or the phrenic nerve, is the most important (PI. XVII. p) ; it springs from the third, fourth, and fifth cervical, and passes down the lower part of the neck, and through the thorax, to supply its own half of the diaphragm. The Cutaneous branches are as follows: the occipitalis minor, to the skin of the occiput; the auriculo-parotidean, to the skin over the parotid gland and the adjacent part of the auricle ; the transversalis colli, to the skin of the front of the side of the neck; the supra-clavicular nerves, to the skin of the lower part of the side of the neck, and upper part of the chest. The Brachial plexus (PL XVII. 1, 2, 3, 4) is formed of the four lower cervical nerves, and of the larger por tion of the first intercostal, called also first dorsal nerve. It is of large size, and is principally for the supply of the upper limb. Its exact mode of arrangement presents many variations, but the following is not un- frequently found : The fifth and sixth nerves join to form a large nerve, which, after a short course, is joined by the seventh; in this manner the upper cord of the plexus is formed. The eighth cervical and the first dorsal then join, to form the lower cord of the plexus. These cords then pass behind the clavicle and subclavius muscle into the axilla, where they become modified in arrangement. From each a large branch arises, and these two branches then join to form a third cord. These three cords have special relations to the axillary artery : the one which lies to its outer side is named the outer cord ; that to the inner, the inner cord; that behind, the posterior cord. These nerves and the cords formed by them give origin to communicating, muscular, cutaneous, and mixed branches. The Communicating branches join the middle and in ferior cervical and first thoracic ganglia of the sympathetic system. The Muscular branches supply the scaleni, longus colli, rhomboid, and subclavius muscles ; the supra and infra-spiuatus muscles, through a branch called supra- scapular ; the serratus magnus, through the posterior thoracic branch ; the greater and lesser pectorals, through the two anterior thoracic branches ; and the subscapu- laris, tercs major, and latissimus dorsi, through the three subscapular branches. The Cutaneous branches arise from the inner cord, and are the lesser internal cutaneous, which ends in the skin of the inner side of the upper arm, and joins the intercosto-humeral ; and the internal cutaneous, which not only sends branches to the skin of the upper arm, but supplies the skin of the inner side of the forearm, both on its anterior and posterior surfaces. The Mixed branches are large and very important: a, The Circumflex, from the posterior cord, supplies the deltoid and teres minor muscles, the skin over the del toid, and the shoulder joint, b, The Musculo- Spiral, also from the posterior cord, supplies the triceps and anconeus, the supinator longus and extensor carpi radialis longior muscles ; and by its external cutaneous branch, the skin of the outer side of the back of the forearm. It then divides into the radial and posterior interosseous branches. The radial passes through the forearm to the hand, and supplies the skin on the back of the thumb, index and middle digits, and radial side of the ring digit. The posterior interosse ous supplies the muscles on the back of the forearm and the articulations of the carpal joints, c, The Musculo-Cutane- ous branch of the outer cord of the plexus supplies the biceps, brachialis anticus, and coraco-brachialis muscles, and ends in an external cutaneous branch, which supplies the skin of the outer side of the forearm, both in front and behind, d, The Ulnar nerve arises from the inner cord, passes through the upper arm, and enters the forearm be tween the inner condyle and olecranon, where it supplies the elbow joint. Here it may easily be compressed, when a pricking sensation is experienced in the course of its dis tribution. In this spot it is popularly called the " funny bone." In the forearm the ulnar nerve supplies the flexor carpi ulnaris and inner part of the flexor profundus digi- torum muscles. In the hand it supplies the muscles of the ball of the little finger, the two inner lurnbricales, the inter- ossei muscles, and the adductor and deep part of the short flexor of the thumb. It also supplies a dorsal cutaneous branch to the back of the hand, and the back of the little and of the ulnar side of the ring digits. Palmar cutaneous branches are also given to the palm and the palmar aspects of the same digits, e, The Median nerve arises by two roots, one from the inner, the other from the outer cord of the plexus. It enters the forearm in front of the elbow joint, supplies, either directly or through its anterior interosseous branch, all the flexors and pronators, except those supplied by the ulnar ; is continued to the hand, where it supplies the abductor, opponens, superficial part of the short flexor of the thumb, and two outer lumbrical muscles. It also supplies a palmer branch to the skin of the palm, and gives digital cutaneous branches to the thumb, index and middle digits, and radial side of the ring digit. The Lumbar plexus, of large size, is situated at the back of the abdominal cavity in the region of the loins, and is formed by the four upper lumbar nerves, which form a series of loop-like interlacements in front of the transverse processes of the lumbar vertebrae. It gives origin to com municating, muscular, cutaneous, and mixed branches. The Communicating branches join the four upper lumbar ganglia of the sympathetic system. Tho Muscular branches supply the quadratus lumborum muscle, and give branches to the psoas. The Cutaneous branches are named a, Ilio-hypoyastric, which gives an iliac branch to the skin of the buttock, and a hypogastric branch to the
skin of the abdomen above the pubic symphysis ; 6 7
