88 UNIVERSITY OF COLORADO STUDIES 



of the olfactory capsule. This is the largest of the cranial nerves. 

 It contains no medullated fibers. 



II. The fibers of the optic chiasma lie sunken in the ventral sur- 

 face Qf the diencephalon. This is common in Urodela, in distinction 

 to the condition in Anura, where the chiasma makes quite a prominence. 

 The fibers of the optic nerve, after crossing, spread out to form the 

 surface layer of white matter of the roof and sides of the mesencephalon. 

 Beyond the chiasma, peripherally, the nerve passes antero-cephalad 

 to its foramen. After leaving the cranium, it turns abruptly down- 

 wards and passes under the ophthalmic branch of V, and after a short 

 lateral course enters the eyeball. The fibers of this nerve are, in the 

 main at least, non-meduUated. Throughout its course, the nerve has 

 a core of gHa cells. These are a direct continuation of the cells hning 

 the preoptic recess, and are a remnant of the cells of the primitive 

 eye-stalk. In Necturus, a more primitive form, the eyes and optic 

 nerve are not so well developed, and the embryonic condition is more 

 strongly suggested than in Plethedon. 



III. The oculo-motor nerve arises from the ventro-lateral aspect 

 of the mesencephalon. At its exit from the brain it is about 75 microns 

 in diameter. It takes a course forward and dorsalward to its foramen, 

 some distance in front of the exit of V. In its foramen it divides into 

 the superior and inferior rami. These pass forward for some distance, 

 one just above and the other just below the ophthalmic branch of V. 

 Then they turn outward to the muscles of the eye. 



IV. The trochlear is the smallest of the cranial nerves, measuring 

 36 microns in diameter and consisting of about twenty fibers. It 

 arises from the line of union between the mid-brain and cerebellum 

 near the mid-dorsal hne. In some Urodela this nerve has not been 

 observed, and in others it has not been traced to its exit from the cra- 

 nium. In Plethedon, though small, its course can be clearly traced. 

 From its place of origin, it takes a course latero-cephalad till it comes 

 in contact with the Gasserian ganghon. It passes forward just be- 

 neath the mesial surface of the ganglion as a small but distinct bundle, 

 and neither gives nor receives fibers from the ganghon. From here 

 it leads forward and upward along the inner surface of the cranium and 



