COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 335 



or cut across them where necessary, thus exposing the musculature of the verte- 

 bral column. Look along the side of this, dorsal to the carotid artery, and note 

 the ventral rami of the spinal nerves emerging at intervals. At about the level 

 of the posterior end of the larynx lies the third cervical nerve in the rabbit, fourth 

 in the cat. As already stated, the nerves thus exposed are the ventral rami only; 

 the dorsal rami are exposed only by more radical dissection, which will not be 

 attempted here. The dorsal rami supply the epaxial musculature. Note the 

 branches of the exposed ventral rami to the muscles of the side of the 

 neck. 



From the ventral ramus of the fourth cervical nerve (rabbit) and fifth 

 cervical nerve (cat) arises the phrenic nerve. It passes posteriorly parallel to 

 the vagus, in the rabbit close to the vertebral musculature. It receives a branch 

 from the fifth (rabbit) or sixth (cat) cervical nerve and then continues posteriorly 

 into the thorax. As it passes the sympathetic ganglia it receives contributions 

 from them. In the thorax it lies at the side of the pericardial sac, just ventral 

 to the root of the lung. Trace it posteriorly and note how it spreads on the 

 surface of the diaphragm. The phrenic nerves are the motor nerves of the dia- 

 phragm; their origin from the cervical nerves shows that the muscles of the 

 diaphragm are derived from cervical myotomes. 



c) The brachial plexus: The ventral rami of the fourth to eighth (rabbit) or 

 fifth to eighth (cat) cervical nerves together with the ventral ramus of the first 

 thoracic nerve are united by intercommunicating branches, called ansae, to form 

 the brachial plexus, which innervates the muscles of the shoulder, breast, fore 

 limb, and diaphragm. The fourth cervical (rabbit), or fifth (cat), take part in 

 plexus only through their contribution to the phrenic nerve. 



To expose the brachial plexus cut through the pectoral muscles near the 

 midventral line and separate the pectoral muscles from the underlying serratus 

 muscle. The plexus lies in the axilla along with the axillary artery and vein. 

 Then cut through the pectoral muscles as near as possible to their insertion on 

 the humerus and separate them from the muscles of the upper arm. In this 

 way the course of the nerves into the fore limb is exposed. 



The connections of the nerves of the plexus are so intricate that it is 

 impossible to describe them. The following points may be noted, however. 

 Rabbit : The fifth cervical immediately sends a branch to the sixth cervical and 

 .then proceeds laterally into the neck muscles. The sixth cervical is a broad 

 nerve which, after communicating with the seventh nerve, passes to the shoulder 

 muscles. The seventh is smaller and after contributing to the eighth likewise 

 innervates the shoulder muscles. The eighth cervical and first thoracic unite to 

 one trunk as they emerge from the vertebral column. From this trunk 

 arise the nerves of the limb. Cat: The sixth cervical has a broad connection 

 with the seventh and then proceeds to the shoulder. The seventh and eighth 

 cervicals and the first thoracic are very stout trunks which are intricately con- 

 nected with each other and from which proceed the nerves of the fore limb. 



