316 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



the roots pass through the cartilage at the side of the cord and unite as they 

 exit from the cartilage. A large number of the most anterior spinal nerves 

 (really the ventral rami of the spinal nerves, the dorsal rami being very slender 

 in the skate) are then seen to unite to form the very large nerve of the brachial 

 plexus previously noted. 



The hypobranchial nerve is a trunk formed by contributions from the occipital 

 nerves and the first spinal nerves. In the dogfishes it may be located as follows. 

 Insert one blade of the scissors in the angle of the jaw and cut back across the 

 gill slits through to the side of the esophagus as was done in an early stage of 

 the dissection (if the same specimen is still being used, this cut will already have 

 been made). Open the flap thus formed and expose the roof of the mouth. 

 Make a longitudinal cut through the mucous membrane of the roof in the median 

 dorsal line. Strip the membrane laterally carrying with it the free dorsal ends 

 of the gill cartilages which will be readily located just lateral to the mid-dorsal 

 line. The visceral branch of the vagus nerve, which was already seen from the 

 other side, is now exposed. It lies along the thin ventral wall of the anterior 

 cardinal sinus. Emerging from the muscle now exposed in the roof of the mouth 

 will be seen the ventral rami of the spinal nerves. In the spiny dogfish two of 

 these (they appear as one but will be found to consist of two on dissecting them 

 toward the median line) pass obliquely toward the visceral branch of the vagus 

 and enter its sheath, thus appearing to join it. The trunk they form can, how- 

 ever, be readily separated from the vagus. This trunk is the hypobranchial 

 nerve. In the smooth dogfish the several most anterior ventral rami unite to 

 form a large trunk, the hypobranchial nerve. This nerve lies just anterior to the 

 nerves of the cervicobrachial plexus to which it contributes branches. It passes 

 to the dorsal side of the visceral branch of the vagus (which may be noted 

 proceeding to the esophagus) posterior to the last visceral pouch, and turning 

 ventrally courses along the floor of the mouth, supplying muscles. 



In the skate the hypobranchial nerve leaves the spinal cord in common with 

 the trunk of the brachial plexus. Locate this trunk again in the anterior wall 

 of the pleuroperitoneal cavity, dorsal to the pleuroperitoneum and just behind 

 the cartilage of the pectoral girdle. The entire trunk with the exception of 

 one branch passes out dorsal to the cartilage to the pectoral fin. This one 

 branch, the hypobranchial nerve, turns forward and is distributed to the muscles 

 of the floor of the mouth. 



The occipital and hypobranchial nerves are probably the homologues of the 

 twelfth or hypoglossal nerve of amniotes, having been taken into the cranium 

 in those forms. The muscles supplied by the hypobranchial nerve are believed 

 to be derived from the most posterior of the head myotomes, and in higher forms 

 become the intrinsic muscles of the tongue. 



6. The ventral aspect of the brain.— Carefully free the brain from the chon- 

 drocranium. First cut through the olfactory tracts and gently lift the ante- 



