CONSIDERATIONS OF THE BRAIN OR ENCEPHALON. 45 



but its real origin is in a mass of gray substance, the genetic 

 nucleus (n. originis], situated within the depths of the mid-brain. 



4. Trochlear Nerve. (AT", trochlearis}. The fourth is a motor 

 nerve to the eye and is the smallest of the cerebral nerves. It 

 may be seen winding forward over the basis pedunculi (Fig. 21). 

 Its apparent origin is from the dorsal surface of the brain stem 

 at the junction of the mid-brain with the hind-brain (the isthmus, 

 Fig. 44) ; this apparent origin cannot be seen in the complete brain. 

 The genetic nucleus of the fourth nerve is located below that 

 of the third in the mid-brain. 



5. Trigeminal Nerve. (N. trigeminus). The trigeminal nerve 

 is a mixed nerve, motor and sensory (Fig. 21). It is attached 

 to the ventral surface of the pons a little above the middle of its 

 lateral border. The small anterior motor root emerges from 

 this point (apparent origin) ; but this is the apparent central termina- 

 tion of the large sensory root, which rises in the semilunar gang- 

 lion (Gasseri) and enters the pons close to the emergence of the 

 motor root. 



6. The abducent nerve (n. abducens] is a motor nerve to 

 the eye. It issues from the pons at its inferior border, or from 

 the transverse groove between the pons and the medulla, just 

 above the pyramid of the medulla and nearly in line with the 

 anterior lateral sulcus (Fig. 21). 



In the transverse groove between the pons and the medulla, 

 lateralward from the root of the sixth nerve, are the roots of the 

 seventh, intermediate and eighth. The seventh is smaller in 

 diameter than the eighth and medial to it in position; the inter- 

 mediate is between these two (Figs. 21 and 45). 



7. The facial nerve (n. facialis} is the motor nerve to the 

 muscles of expression (Figs. 21 and 45). Rising from a nucleus 

 in the pons, its emergence from the transverse groove between the 

 medulla and pons constitutes its apparent origin. The inter- 

 mediate nerve (n. intermedius) is so closely associated with the 

 facial nerve that many regard it as the sensory root of that nerve; 

 but the intermediate nerve is in reality a mixed nerve with efferent 

 fibers of vasodilator, secretory and trophic functions and afferent 

 fibers whose function is taste. The efferent fibers rise from the 



