90 UNIVERSITY OF COLORADO STUDIES 



One branch (infra-orbital) breaks up back of the eyeball, and the 

 other (maxillary) passes forward a little above the maxilla. The 

 mandibular passes off laterally and soon curves ventralward to the 

 maxilla, where it breaks up into three branches. Two of these remain 

 in the upper jaw and pass forward well into the snout, one just external 

 to, and the other just internal to the maxilla. The distribution of this 

 nerve in Plethedon seems to be exceptional. I am unable to find an 

 account of any other form in which this large part of this nerve remains 

 in the upper jaw. These two branches may possibly represent a part 

 of the maxillary ramus. The third branch, about equal in size to the 

 other two combined, passes ventralward into the lower jaw. 



VI. The abducens arises, as usual, from near the mid-ventral line 

 of the medulla, a short distance back of VIII. It is a slender never, 

 raveaging about 65 microns in diameter. It takes a course forward 

 and dorsal ward and passes just below the ganglion of VII- VIII, and 

 below the Gasserian ganglion. In this respect Plethedon seems to be 

 exceptional, inasmuch as this nerve is usually described in amphibia 

 as passing through the Gasserian ganglion. It makes its exit from 

 the cranium in front of the ophthalmic branch of V. 



VII, VIII. The facial and auditory arise by a common trunk, from 

 the lateral surface of the medulla just back of V. The combined 

 trunk, after a short course in an antero-lateral direction, enters its 

 ganglion, which is composed of the geniculate and auditory ganglia. 

 From the ganglion, four nerve trunks make their exit, two of which 

 belong to VII and two to VIII. 



The branches of VII are the palatine and hyo-mandibular. The 

 palatine passes forward in the roof of the mouth, to which it gives off 

 branches. Its union with the ventral terminal branch of V has been 

 described under V. The hyo-mandibular trunk of VII passes out- 

 ward under the ear capsule and then curves downward to the lower 

 jaw. A little distance from its foramen, it gives off a branch back- 

 ward which, after a short course, unites with IX. This anastomosing 

 branch between VII and IX is quite common in amphibia. A branch 

 of VII uniting with the Gasserian ganghon is uniformly described for 

 amphibia. I failed to find this branch in Plethedon. 



