814 NEEVOUS SYSTEM OF THE HORSE 



the left vagus and a cardiac nerve, and crosses over the dorso-cervical vein. It 

 then parts company ^dth 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 conununicans, 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, i. 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 exceptiod 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 thoracic 

 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 i^ 



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.^ 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 



' In order to examine the plexus and the origins of its chief branches conveniently and with 

 as Uttle 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. 



' The loop may be absent or double. 



