334 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



may be removed in order to expose the latter. In following one nerve, adjacent 

 nerves must not be destroyed. 



i. The spinal nerves, the sympathetic system, and the vagus. 



a) Cervical portion of the sympathetic and the vagus: Locate the vagus nerve 

 at a point near the larynx. It lies alongside the carotid artery. The nerve 

 crossing the vagus near the larynx and giving off branches into the sternohyoid, 

 sternothyroid, and related muscles is the descending branch of the twelfth or 

 hypoglossal nerve. 



Rabbit: The vagus nerve and the cervical part of the sympathetic trunk lie 

 together on the dorsal surface of the carotid artery. The vagus is larger and 

 more lateral. To the medial side of the sympathetic trunk posterior to the 

 larynx may be separated a delicate nerve, the cardiac branch of the vagus (depressor 

 nerve of the heart) . Trace the sympathetic posteriorly. Just in front of the sub- 

 clavian artery it enters a ganglion, the inferior cervical ganglion. From this 

 ganglion cords pass to either side of the subclavian artery, forming the ansa sub- 

 clavia, and unite again to another ganglion, the first thoracic ganglion, situated 

 posterior and dorsal to the artery. 



Cat: The sympathetic trunk is inseparably bound with the vagus, the two 

 forming a large vago-sympathetic trunk coursing lateral to the carotid artery and 

 bound with it by a common sheath. Trace it caudad. Just in front of the first 

 rib branches of sympathetic origin arise from the trunk and proceed toward the 

 esophagus. Shortly posterior to this point the sympathetic separates from the 

 vagus and generally enters a ganglion, the middle cervical ganglion, which lies 

 in contact with the vagus. From this ganglion cords pass on either side of the 

 subclavian artery, forming the ansa subclavia, and proceeding directly dorsally 

 unite to form a large ganglion, the inferior cervical ganglion, which lies against 

 the neck muscles between the heads of the first and second ribs. 



From the inferior cervical ganglion in both animals cardiac branches pass to 

 the heart. The conspicuous nerve lying lateral to the vagus is the phrenic nerve 

 or nerve of the diaphragm. The right vagus just after passing ventral to the 

 subclavian artery gives off the recurrent or posterior laryngeal nerve which runs 

 anteriorly along the side of the trachea to the larynx. The left recurrent nerve 

 arises much farther posteriorly from the left vagus. 



b) The anterior cervical spinal nerves: The spinal nerves emerge from the 

 spinal cord in pairs between successive vertebrae, passing out through the inter- 

 vertebral foramina. Those of the cervical region are called the cervical nerves; 

 there are eight pairs of them. The ventral rami of the first four cervical nerves 

 are loosely united with each other to form the cervical plexus; the last four 

 together with the first thoracic form the brachial plexus. As the first two are 

 small and more or less mingled with the posterior cranial nerves they will not 

 be studied at this stage of the dissection. 



To expose the cervical nerves, pull the muscles which are inserted on the 

 anterior end of the sternum (sternomastoid, sternohyoid, sternothyroid) laterally 



