730 THE NERVOUS SYSTEM OF THE DOG 



tinues along the trachea internal to the recurrent nerve. At the thoracic inlet it 

 communicates with the inferior cervical ganglion and continues backward to unite 

 with the vagus at or near the point of origin of the recurrent nerve. Its collateral 

 filaments supply the trachea and concur with the inferior pharyngeal branch in 

 forming a plexus on the cervical part of the oesophagus which innervates that tube.^ 

 The depressor nerve is a very delicate filament which arises usually from the supe- 

 rior laryngeal nerve and accompanies the vago-sympathetic trunk to the thorax; 

 its fibers reach the heart through the cardiac nerves. The pulmonary and pos- 

 terior oesophageal plexuses are highly developed. 



The hypoglossal nerve gives off a long branch (R. descendens) which runs 

 downward and backward across the pharynx and larynx, communicates with the 

 ventral branch of the first cervical nerve, and supplies the thyro-hyoid, sterno- 

 hyoid, and sterno-thyroid muscles. 



The spinal nerves number thirty-six or thirty-seven on either side, and com- 

 prise eight cervical, thirteen thoracic, seven lumbar, and five or six coccygeal. 



The brachial plexus (Fig. 487) is derived from the ventral branches of the last 

 four cervical and first thoracic nerves; the root supplied by the fifth cervical nerve 

 is very small. The roots unite at the lower border of the scalenus. The more 

 important special features in the arrangement of the nerves which emanate from 

 the plexus are as follows: 



The musculo-cutaneous nerve passes between the coraco-brachialis and the 

 brachial artery and descends in the arm in front of the artery. At tlie shoulder 

 joint it gives off branches to the biceps and coraco-brachialis, and in the low^r third 

 of the arm is connected with the median nerve by an oblique branch. It terminates 

 near the elbow by dividing into a branch for the brachialis and a small cutaneous 

 nerve which passes down over the inner face of the elbow and, inclining a little 

 forward, descends over the deep fascia of the forearm to the carpus. 



The radial nerve descends behind the ulnar nerve, gives branches to the ex- 

 tensors of the elbow, dips in between the internal head of the triceps and the ac- 

 cessory head of the anconeus, winds around the arm, and divides between the brach- 

 ialis and the external head of the triceps into two branches. The deep branch 

 (R. profundus) supplies the extensor and supinator muscles on the forearm. The 

 superficial branch (R. superficialis) emerges upon the flexor surface of the elbow 

 and divides into two })ranches which terminate by supplying two dorsal digital 

 nerves to each digit, except the fifth, which receives its outer dorsal nerve from the 

 ulnar. The inner branch descends along the inner side of the cephalic vein to the 

 carpus, where it divides into dorsal digital nerves for the first digit and the inner 

 side of the second. The outer branch is much larger. It runs downward on the 

 middle of the front of the forearm and su]:>plies the remaining dorsal digital nerves 

 except that to the outer side of the fifth digit. 



The ulnar nerve is as large as or larger than the median, with which it is united 

 for some distance. At the distal third of the arm it separates from the median 

 and passes over the internal epicondyle of the humerus. At the proximal part of 

 the forearm it gives off the dorsal branch (R. dorsalis), which supplies cutaneous 

 twigs to the dorso-lateral surface of the lower part of the forearm and carpus and 

 terminates as the external dorsal digital nerve of the fifth digit. Descending under 

 cover of the flexor carpi ulnaris, the ulnar nerve inclines inward under the tendon 

 of insertion of that muscle and divides into superficial and dee]) branches. The 

 superficial branch (R. su])erficialis) runs downward along the outer l)order of the 

 flexor tendons, gives off the external volar digital nerve of the fifth digit (N. dig. 

 vol. lat. dig. V), and a branch (N. met. vol. IV) which descends in the space between 

 the fourth and fifth metacarpal bones and unites with the deep branch. The deep 



' Lesbre terms this the tracheo-CEsophageal branch, and considers that it must be regardetl 

 as an accessory or internal recurrent nerve. 



