314 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



ti) The eighth or auditory nerve: The auditory nerve is a pure somatic sensory 

 nerve extending from the internal ear to the brain. It enters the anterior end 

 of the medulla and is there mingled with the roots of the fifth and seventh nerves. 

 Follow it into the internal ear on the side opposite that on which the hyoman- 

 dibular was dissected. Note its branches to each ampulla and the fanlike arrange- 

 ment of the branchlets to the crista of each ampulla. The auditory nerve also 

 collects a number of branches from the walls of the sacculus and utriculus. It 

 carries impulses for hearing and equilibration into the acustico-lateral area of 

 the medulla to which it will be found to be attached. There is no motor nerve 

 corresponding to the auditory nerve. 



i) The ninth or glossopharyngeal nerve: This nerve passes through the floor 

 of the middle of the ear capsule (where it is likely to be mistaken for a part of 

 the auditory), parallel to the hyomandibular nerve. Pare away as much of the 

 ear capsule as is necessary to reveal it. Find its attachment to the medulla 

 posterior to the auditory nerve. Trace it out of the ear capsule. Just before it 

 exits from the ear capsule it bears a swelling, the petrosal ganglion. Insert a 

 knife blade into the second (first typical) gill slit (in the skate into the dorsal 

 wall of the corresponding visceral pouch) and slit the gill cleft open dorsally. 

 The petrosal ganglion will now be seen to be located near the upper limits of the 

 cleft. Dissect the nerve from the ganglion toward the gill slit. It very soon 

 divides into three branches, two smaller anterior ones, and a larger posterior 

 one. The most anterior branch is the pretrematic branch and passes to the 

 anterior wall of the visceral pouch to which it is a sensory nerve. The second 

 branch is posterior to the pretrematic branch; it is named the pharyngeal branch 

 and is a sensory nerve to the mouth cavity. (This branch appears to be lacking 

 in the skate.) The third and largest is the post-trematic branch. It passes to 

 the posterior wall of the visceral pouch and is both sensory and motor, its motor 

 components supplying the muscles of the third visceral arch. The glossopharyn- 

 geal nerve is the nerve of the second visceral pouch and of the third visceral arch. 



j) The tenth or vagus nerve: The vagus nerve is the very large trunk passing 

 through the posterior border of the ear capsule. It is attached to the sides of 

 the posterior part of the medulla. Dissect it out and follow its course. It 

 passes to the anterior cardinal sinus, the wall of which is formed in the dogfishes 

 of a tough membrane. Open up the anterior cardinal sinus by a deep cut through 

 the muscles medial to the visceral pouches. Follow the vagus nerve into the 

 anterior cardinal sinus. In the dogfishes it divides into two trunks at the point 

 where it penetrates the tough wall of the sinus. The medially situated trunk 

 is the lateral branch of the vagus and passes posteriorly internal to the lateral 

 line whose canal it supplies. The lateral trunk is the visceral branch of the 

 vagus, which continues along the anterior cardinal sinus. In the skate the 

 vagus runs for a short distance in the sinus before dividing into a more dorsal 

 lateral branch, which passes posteriorly internal to the lateral line canal to which 



