182 WILLIAM F. ALLEN 



The other three series confirm the results recorded above. Also 

 figure 19 shows more degenerated fibers in the left mesen- 

 cephalic root than were present in the right. It is from a section 

 at about the same level as figure 12, but from another experi- 

 ment, where the left mesencephalic root was severed behmd the 

 inferior colliculus. This section is cephalad of the lesion and 

 the degenerated fibers are ascending fibers, namely, the same 

 sensory fibers as were cut in experiment no. 58, which had their 

 cells of origin in the semilunar ganghon. 



It is of interest to record in connection with figure 13 that the 

 'right trochlear nerve root (7 F) contains more degenrated nerve 

 fibers than the left. This section shows the trochlear roots 

 after the fibers had crossed, so that the right root is really the 

 left, which from its position (fig. 20, IV) would be severed with 

 the trigeminal roots in expermient no. 58. This might signify 

 that the trochlear nerve in the guinea-pig contains some scattered 

 muscle sense ganglion cells, such as NichoUs described for the 

 oculomotor nerve in sharks and in the frog, the central processes 

 of which were severed in this experiment. 



DESCENDING FIBERS IX THE MESENCEPHALIC ROOT 



1 . Distribution of the nervus trigeminus in the guinea-pig 



It is obvious before much progress can be made in the solution 

 of the distribution of the descending mesencephalic root fibers 

 that an accurate knowledge should be obtained of the relation- 

 ships of the various branches of the trigeminal nerve. With 

 this in view, a careful dissection of this nerve was made in the 

 guinea-pig. Also the physiological action of these nerves was 

 tested by stimulating them with a weak induction current, the 

 results of which will be recorded briefly before considering the 

 experiments for determining the distribution of the descending 

 mesencephahc root fibers. 



As previously st'ated, the semilunar ganglion (fig. 3, S.G.) is 

 situated within the skull directly above a large foramen in the 

 alisphenoid (fig. 4, S.F.). The ganglion is more or less elliptical 

 in shape, having its long axis parallel with the axis of the animal. 



: 



