21 



4. The palatine branch springs from the base of the hyoman- 

 dibular trunk inside the hyomandibular canal. It passes outward 

 and forward, dividing into numerous branches which innervate the 

 mucous membranes of the mouth. It can be traced completely later. 



One or more small nerves proceeding to the pseudobranch and 

 anterior wall of the spiracle arise near the point of origin of the 

 palatine branch. 



The otic nerve, passing from the root of the facial nerve to the 

 postorbital canal, is not likely to be found in this dissection. 



Observe the enlargement near the base of the hyomandibular 

 trunk, and within the cartilaginous canal, the geniculate ganglion. 

 The gasserian ganglion, a component of the trigeminal nerve, lies in 

 the ventral portion of the trigeminal-facial root, and can now be 

 located. 



The auditory nerve arises close behind the ventral division of 

 the trigemino-facial root. The root of the auditory nerve encloses 

 a large auditory ganglion. A vestibular nerve arises from the an- 

 terior end of the auditory ganglion and passes into the ear capsule, 

 innervating the upper part of the utriculo-sacculus and the ampullae 

 of the anterior and horizontal canals. From the posterior part of 

 the ganglion nerves pass to the ventral part of the sacculus and the 

 ampulla of the posterior canal. Trace these branches as thor- 

 oughly as possible. 



The abducens nerve emerges from the cranium under and close 

 to the origin of the posterior rectus muscle, into which muscle it 

 enters. To expose this nerve the trigeminal, facial and auditory 

 nerves must be lifted and cut as they pass through the wall of the 

 cranium. It can be traced obliquely backward and inward, through 

 a long canal, to its origin near the mid-line of the ventral surface 

 of the medulla. 



The glossopharyngeal nerve passes through the base of the ear 

 capsule from the side of the medulla to the upper end of the first 

 branchial pouch. A ganglionic enlargement is found near where it 

 emerges from the cartilage. Outside the cranium the glossopharyn- 

 geal divides into a pretrematic branch, passing down in front of 

 the first gill pouch, and a posttrematic branch running behind the 

 pouch. The pretrematic branch quickly sends off a pharyngeal 

 nerve which runs antero-ventrally to the roof of the pharynx. The 

 pretrematic and posttrematic branches can be followed along the 

 gill-arch to the ventral side of the pharynx. A fourth branch of 

 the glossopharyngeal, the supratemporal, springs from the dorsal 

 side of the ganglion; passing through the ear capsule it runs to the 

 dorsal surface of the head, where it is distributed to the sense organs 

 of a short section of the lateral line canal. This small nerve can 

 be demonstrated by carefully separating the muscles and perichon- 

 drium from the posterior surface of the auditory capsule. 



The vagus nerve (or pneumo gastric) arises by an extensive 

 series of roots from the side of the medulla. An easily distinguished 

 ribbon-like portion of the root, the lateral line root, runs forward 

 as far as the root of the glossopharyngeal. Note the canal by which 

 the vagus leaves the cranium, and trace the nerve along the inner 

 side of the anterior cardinal vein. 



The principal branches of the vagus are: 



1. The supratemporal branch, a small nerve running dorsal 

 through the posterior part of the ear capsule to the lateral line 

 canal and other sense organs of the head. It will be found near the 

 supratemporal branch of the glossopharyngeal. 



