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 liackward 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 correspontling 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 band 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 by the sixth cervical nerve is quite small. Each of the 

 three chief roots, i. e., those from the last two cervical and the first thoracic nerve, 

 is connected with the sjaiipathetic by a ramus communicans. 



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

 limb, but some arc distributed on the chest-wall. The names of the branches, and 

 tiieir arrangement so far as they can be conveniently examined before removal of 

 the fore limb, are as follows:^ 



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

 ward and disa])pears between the su]oraspinatus and sul)scapularis. 



2. The much smaller subscapular nerves (Nn. subscapulares), usually two 

 primary trunks, arise close behind tlic 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 numl)er, 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 between the divisions of the deep pectoral to 

 suppl}' th'. 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. mAisculocutaneus) arises from the an- 

 terior part of the ])lexus and j^asscs 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 whicli the artery lies. One or two branches to the pectoral muscles 

 are given off from the nerve or the loop. 



."). The median nerve (N. medianus) is usually the largest branch of the bra- 

 chial ])lexus. it lies at first above the brachial artery, then passes between 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 fonns with the musculo-cutaneous nerve. 



G. The ulnar nerve (N. ulnaris) arises Avith the median by a short common 

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

 by the radial nerve. 



7. The radial nerve (\. radialis) arises from the posterior part of the plexus 

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

 the origin of the sul)sca]iular artery and the lower part of the teres major, and 



1 In order to examino tlio plexus and tlic origins of its cliiof branches conveniently and with 

 as little disturbance of relation as possible, the subject should be suspended in imitation of the 

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



