THE MAMMALIAN NERVOUS SYSTEM 69 



sections find the sensory V root in the pons region (see the lefer- 

 ences in Section 62 (7)), and follow its fibers backward into the 

 spinal V tract. Only a part of the sensory V root-fibers enter 

 this tract. Others enter the chief sensory V nucleus, which lies 

 dorsally of the spinal V tract at the level of entrance of the fibers 

 of the V nerve and forward (rostrally) of this level. The chief 

 sensory nucleus of the V nerve and the spinal V nucleus (sub- 

 stantia gelatinosa of Rolando), which accompanies the spinal 

 V tract, are the terminal nuclei of the general cutaneous com- 

 ponents of the cranial nerves. These nuclei and the spinal V 

 tract should be located and entered in the drawings of all sec- 

 tions in which they occur. 



A few general cutaneous fibers enter the spinal V tract and 

 its nucleus from the IX and X roots also, but these are usually 

 not evident in the sections. The motor nucleus of the V nerve 

 will be seen lying medially of the chief sensory V nucleus and it 

 may be drawn in at this time. Golgi sections show that some 

 peripheral sensory V fibers end in the motor V nucleus, thus 

 providing a direct reflex connection between the skin of the 

 face and mouth and the jaw muscles. Neurons of the spinal V 

 nucleus effect the connection between the peripheral cutaneous 

 fibers of the spinal V tract and the motor nuclei of the VII, IX, 

 and X nerves. 



74. Trigeminal lemniscus. The ascending secondary fibers 

 from the chief sensory V and spinal V nuclei to the somesthetic 

 nuclei of the thalamus (lateral and related nuclei) form 

 two tracts, both of which are called the trigeminal lemniscus. 

 Their fibers cannot easily be traced in either gross or micro- 

 scopic preparations. 



For diagrams showing the positions of the V nuclei and their 

 connections with other centers in the brain, see Bailey ('16), 

 Fig. 344, p. 512; Herrick ('18), Figs. 64, 71, 73, 75, 77, 78, 81; 

 Gray ('18), Figs. 696-698; Morris ('14), Fig. 654, p. 828; Quain 

 ('09), Vol. 3, Pt. 1, Figs. 170, 171, 172, 175, 176; Villiger ('12), 

 Figs. 163, 164. 



75. The Mesencephalic V root. The fibers of this root can 

 easily be recognized in the microscopic sections; see Herrick 

 ('18), Fig. 75, and the references cited in Section 62 (8) and (9). 

 The root-fibers for the motor and chief sensory V nuclei form a 



