696 NERVOUS SYSTEM OF THE HORSE 



gradually to the ventral face of the vein. On the left side the nerve enters the 

 thorax between the left brachial and inferior cervical arteries, and then runs its 

 entire course in the mediastinum. In the anterior mediastinum it lies along the 

 brachiocephalic artery ventral to the left vagus and cardiac nerves, and crosses 

 over the dorso-cervical vein. It then 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 I'ty dividing into several branches which 

 are distributed to the corresponding part of the diaphragm. 



The Brachial Plexus 



The brachial plexus (Plexus brachialis) (Fig. 433) results from anastomoses 

 established between the ventral branches of the last three cervical and first two 

 thoracic nerves. It appears as a thick, wide })and between the two parts of the 

 scalenus muscle, and is covered by the anterior deep pectoral and subscapularis 

 muscles. The largest root of the plexus is derived from the first thoracic nerve, 

 while the root supplied liy the sixth cervical nerve is quite small. Each of the 

 three chief roots, /. c, those from the last two cervical and the first thoracic nerve, 

 is connected with the sympathetic by a ramus communicans. 



The branches emanating from the plexus go for tlie most part to the thoracic 

 liml), 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 rem.oval of 

 the fore limb, are as follows:^ 



1. The large suprascapular nerve (X. suprascapularis) turns outward and for- 

 ward and disappears between tlie supraspinatus and su])scapularis. 



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 lower third of the subscap- 

 ularis. 



3. The anterior thoracic or pectoral nerves (N. 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 out Ijetween the divisions of the deep pectoral to 

 supply the superficial pectoral, giving a twig usually to the posterior deep muscle. 

 The latter receives one or two other nerves. 



4. The musculo-cutaneous nerve (N. niusculocutaneus) arises from the an- 

 terior part of the plexus and passes over the outer face of the brachial artery, below 

 which it is connected by a large but short branch with the median nerve, thus form- 

 ing a loop in which the artery lies. One or two l)ranches 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 lies at first al:>ove the brachial artery, then passes l:)etween the 

 artery and vein to reach the anterior border of the former. It is easily recognized 

 by its large size and the loop which it forms v.'ith the musculo-cutaneous nerve. 



6. The ulnar nerve (N. ulnaris) arises with tlie median by a short common 

 trunk. It descends behind the brachial artery and is accompanied a short distance 

 by the radial nerve. 



7. The radial nerve (N. radialis) aris(^s from the posterior part of the plexus 

 and is sometimes the largest branch. It descends behind the ulnar nerve over 

 the origin of the subscapular artery and the lower part of the teres major, and 



1 In order to examine the plexus and tlu> origins of its cliicf l)ranches conveniently and with 

 as little distiirbanee of relation as possible, the subject should ho suspended in imitation of the 

 natural position and the fore limlj abducted as much as is necessary. 



