Johnston, TJye Radix Mesencephalica Trigemini. 619 



deep lateral groove in the wall of the fourth ventricle, the sulcus 

 limitans of His. Lateral to this the sensory zone (acustic and tri- 

 geminal centers) of the medulla is continued dorsally by the develop- 

 ing cerebellum. Medially from the sulcus limitans is, of course, the 

 thick brain base containing the motor centers. These relations are 

 clearly seen in the model (Fig. 13) which includes a part of the 

 medulla oblongata, the cerebellum and a part of the midbrain. 



The course of the mesencephalic bundle from the sensory root up 

 to the velum medullare is shown in Fig. 14, A, B, C, D. Fig. 15 

 shows the actual appearance of the section of which Fig. 14 A is an 

 outline sketch. It will be seen that the sensory root enters the brain 

 lateral to the sulcus limitans, while the motor root bends mesad into 

 its nucleus in the base. The mesencephalic bundle lies in the midst 

 of the sensory root fibers as a distinct compact bundle. As it passes 

 forward (Fig. 14 B, C, D) it gradually rises above the sulcus limitans 

 until at the velum medullare it assumes the typical relations to the 

 IVth nerve described above. At the level shown in Fig. 14, C, and 

 caudal to that are found the large cells which later occupy the locus 

 coeruleus. They are all located lateral and dorsal to the dense layer 

 of central gray which surrounds the sulcus limitans. The bundle and 

 the cells related to it are therefore wholly in the dorsal or sensory 

 zone of the brain as in fishes, amphibians and reptiles. 



The relation of the mesencephalic bundle to the cerebellar fibers 

 of the trigeminus is shown in three other sketches (Figs. 16, lY, 18) 

 taken from sections between those drawn in Figs. 14, A and B. 

 These sketches show that in the early embryo the mesencephalic 

 bundle is intimately associated with the ascending sensory root of 

 the trigeminus, as was to be expected from its relations in lower 

 vertebrates. It is also evident from all these figures that in the 

 young embryo the mesencephalic bundle has not the remotest apparent 

 relation to the motor root or nucleus. That these relations become 

 so greatly modified in the adult is due to a number of influences 

 among which are probably the great growth of the cerebellum and 

 the vestibular nuclei related to it and the great size of the corpus 

 restiforme, all of which tend to crowd the sensory bundles of the 

 trigeminus closer to the motor nucleus. Another important factor is 



