MESENCEPHALIC ROOT 199 



glion and the motor cells of the trigeminal motor nucleus, agrees 

 with Johnston and Willems that they are sensory rather than 

 motor and favors Johnston's hypothesis that they represent 

 neural crest cells, which were not extruded or possibly were 

 extruded and later pulled back, when the medullary folds of the 

 midbrain rolled up to form a tube. So far as known, there 

 would be no need in the trigeminal nerve for a special motor 

 nucleus, like the salivatory nuclei of the facial and glossopharjnri- 

 geal nerves; the trigeminal motor nucleus should be fully capable 

 of regulating the contractions of the masticator muscles. 



Granting that the descending mesencephalic root fibers are 

 sensory, it follows from their distribution that they are not 

 carriers of cutaneous sensations, else they would be distributed 

 to the maxillary, ophthalmic, and sensory branches of the man- 

 dibular nerve; hence they must be muscle sense fibers. In 

 fact, it is difficult to conceive how muscle sense, say from the 

 masseter muscle, which surely must possess muscle sense endings, 

 could reach the trigeminal nerve other than through the masseter 

 branch of the nervus masticatorius, which fibers we have shown do 

 not enter the semilunar ganglion nor the trigeminal sensory root. 



There can be no question but that the ascending mesencephalic 

 root fibers are sensory. Since they were not found in the dorsal 

 part of the sensory root, they probably did not come from the 

 ophthalmic and maxillary (general cutaneous) divisions of the 

 trigeminal nerve. More than likely, the ascending mesen- 

 cephalic root fibers are also carriers of muscle sense. Inasmuch 

 as no descending mesencephalic root fibers were present in the 

 nervus mandibularis proper in the guinea-pig and Willems 

 found no chromatolysis of the mesencephalic root nucleus cells 

 after severing the mylohyoid and digastric nerves, it is quite 

 possible that these ascending fibers came from the mylohyoid 

 and digastric muscles, and entered the main mandibular nerve 

 through the nervus mylohyoideus. It should be noted that 

 some of the degenerated ascending fibers seen in the mesen- 

 cephalic root as it passes between the trigeminal motor and 

 sensory nuclei may be cutaneous fibers and collaterals . from the 

 sensory nucleus going to the motor nucleus. 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 30, NO. 2 



