814 NERVOUS SYSTEM OF THE HORSE 
the left vagus and a cardiac nerve, and crosses over the dorso-cervical vein. It 
then parts company with the vagus, passes over the upper part of the pericardium, 
and runs backward in the posterior mediastinum to reach the tendinous center of 
the diaphragm considerably to the left of the median plane. Each nerve is usually 
connected near its origin with the first thoracic ganglion of the sympathetic by a 
ramus communicans, and each terminates by dividing into several branches which 
are distributed to the corresponding part of the diaphragm. 
THE BRACHIAL PLEXUS 
The brachial plexus (Plexus brachialis) (Fig. 558) results from anastomoses 
established between the ventral branches of the last three cervical and first two 
thoracic nerves. It appears as a thick, wide band between the two parts of the 
scalenus muscle, and is covered by the anterior deep pectoral and subscapularis 
muscles. Each of the three chief roots, 7. e., those from the last two cervical and the 
first thoracic nerve, is connected with the sympathetic by a ramus communicans, 
The root derived from the sixth cervical nerve is very small, while the ventral branches of 
the seventh and eighth cervical nerves go almost entirely to the plexus. The first thoracic nerve 
furnishes the largest root; its whole ventral branch goes to the plexus with the exception of its 
small first intercostal branch. The root from the second thoracic nerve is small, since most of its 
ventral branch goes to form the second intercostal nerve. 
The branches emanating from the plexus go for the most part to the thoracie 
limb, but some are distributed on the chest-wall. The names of the branches, and 
their arrangement so far as they can be conveniently examined before removal of 
the forelimb, are as follows:! 
1. The large suprascapular nerve (N. suprascapularis) arises from the anterior 
part of the plexus, passes ventro-laterally, and disappears between the supraspinatus 
and subscapularis muscles. 
2. The much smaller subscapular nerves (Nn. subscapulares), usually two 
primary trunks, arise close behind the suprascapular, run backward a short dis- 
tance, and divide into several branches which enter the distal third of the subscap- 
ularis muscle. 
3. The anterior thoracic or pectoral nerves (Nn. pectorales craniales), three or 
four in number, arise from the anterior part of the plexus and from the loop formed 
by the musculo-cutaneous and median nerves. One enters the anterior deep 
pectoral muscle. Another passes between the divisions of the deep pectoral to 
supply the superficial pectoral and brachiocephalicus, giving a twig usually to the 
posterior deep muscle. The latter receives one or two other nerves. 
4. The musculo-cutaneous nerve (N. musculocutaneus) arises from the ante- 
rior part of the plexus and descends over the lateral face of the brachial artery, 
below which it is connected by a large but short branch with the median nerve, 
thus forming a loop in which the artery lies.2. One or two branches to the pectoral 
muscles are given off from the nerve or the loop. 
5. The median nerve (N. medianus) is usually the largest branch of the bra- 
chial plexus. It descends over the insertion of the scalenus, crosses the medial 
face of the brachial artery, and reaches the anterior border of that vessel. It is 
easily recognized by its large size and the loop which it forms with the musculo- 
cutaneous nerve. 
6. The ulnar nerve (N. ulnaris) arises with the median by a short common 
1 Tn order to examine the plexus and the origins of its chief branches conveniently and with 
as little disturbance of relation as possible, the subject should be suspended in imitation of the 
natural position and the forelimb abducted as much as is necessary. In doing this some disturb- 
ance of the position and relations of the nerves and vessels takes place. The brachial vessels are 
drawn away from the chest-wall so that their lateral surface now faces forward, and the nerves 
are similarly affected. 
2 The loop may be absent or double. 
