THE NERVOUS SYSTEM OF THE DOG 853 
other animals, has a peculiar 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 cesophagus which 
innervates that tube.1. 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-sympathetie 
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 cesophageal 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 
brachialis 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-cesophageal branch, and considers that it must be regarded 
as an accessory or internal recurrent nerve. 
