THE NERVOUS SYSTEM OP THE DOG 853 



other animals, has a pecuhar arrangement. It runs back near the dorsal border of the 

 thyroid cartilage, gives a branch to the crico-arytenoideus dorsalis as it passes over 

 that muscle, and continues along the trachea medial to the recurrent nerve. At the 

 thoracic inlet it communicates with the posterior 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 posterior 

 pharyngeal branch in forming a plexus on the cervical part of the oesophagus which 

 innervates that tube.^ The recurrent nerves present no remarkable special features. 

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

 terior laryngeal nerve and is incorporated in the vago-sympathetic trunk to the 

 thorax. Here the nerve separates from the ventral border of the vago-sympathetic 

 trunk. The right nerve passes backward between the anterior vena cava and the 

 trachea, inclines to the left, and reaches the medial surface of the aortic arch. • The 

 left nerve passes backward ventral to the vagus and across the lateral (left) face 

 of the aortic arch. Both nerves give filaments to the cardiac nerves and to the 

 aorta and pulmonary artery. Their terminal branches pass between these vessels 

 and enter the wall of the heart. The pulmonary and posterior (Esophageal 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. 616) 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 ventral 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 the shoulder 

 joint it gives off branches to the biceps and coraco-brachialis, and in the distal 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 medial 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 

 extensors of the elbow, dips in between the medial head of the triceps and the 

 accessory head of the anconeus, winds around the arm, and divides between the 

 brachiahs and the lateral 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 branches which terminate by supplying two dorsal digital 

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

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

 carpus, where it divides into dorsal nerves for the first digit and the medial side of 

 the second. The lateral branch is much larger. It descends on the middle of the 

 front of the forearm and supplies the' remaining dorsal digital nerves except that to 

 the lateral 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 medial epicondyle of the humerus. At the proximal part of 



1 Lesbre terms this the tracheo-oesophageal branch, and considers that it must be regarded 

 as an accessory or internal recm-rent nerve. 



