180 DISSECTION OF BRAIN. 



and its course in the orbit is described in p. 56. In the eyeball it ends in 

 the retina. 



The THIRD NERVE ( 3 ), motor nerve of the eyeball, is round and firm, 

 and is attached by a slanting line of threads to the inner surface of the 

 cerebral peduncle, near the locus perforatus, and close in front of the pons 

 Varolii. 



Deep origin. The deep origin is uncertain. According to Stilling, 1 the fibres of 

 the nerve pierce the peduncle, passing through the locus niger, and enter a nu- 

 cleus of gray substance near the floor of the aqueduct of Sylvius. 



The FOURTH or TROCHLEAR NERVE ( 4 ) cannot be followed backwards 

 at present to its origin. It is the smallest of the cranial nerves, and issues 

 from the valve of Vieussens over the fourth ventricle (fig. 68, *). The 

 nerve appears between the cerebrum and the cerebellum, on the side of 

 the crus cerebri ; and is then directed forwards to enter an aperture in the 

 free edge of the tentorium cerebelli near the posterior clinoid process. 



Deep origin. In entering the valve of Vieussens, filaments of the nerves of oppo- 

 site sides cross and communicate. Each divides into two parts : the anterior enters 

 a nucleus of gray matter on the side of the aqueduct of Sylvius ; the posterior 

 joins a nucleus (upper trigeminal) near the top of the fourth ventricle (Stilling). 



The FIFTH or TRIGEMINAL is the largest of all the cranial nerves, and 

 consists of two roots, ganglionLc. or sensory, and aganglionic or motor, 

 which are partly blended in one trunk beyond the ganglion. 



The nerve is attached to the side of the pons YaroliL. nearer the upper 

 than the lower border ( 5 ). The small or aganglionic root is highest, and 

 is separated from the large one by two or three of the transverse fibres of 

 the pons. Both roots pass outwards through an aperture in the dura 

 mater, above the petrous part of the temporal bone, and are blended in the 

 peculiar manner stated in page 31. 



Both roots penetrate the fibres of the pons, and are connected wjthjiu- 

 clei in the floor of the fourth ventricle. 



Deep origin. The large root divides into two parts near the mass of gray matter 

 called locus cseruleus (p. 217). One piece arises from the gray matter near the 

 locus creruleus (upper trigeminal nucleus) ; and the other, from a deeper nucleus, 

 lower trigeminal, opposite the lower border of the pons, within the fasciculus teres 

 (Stilling). 



The small root begins with the fourth nerve in the upper trigeminal nucleus near 

 the top of the fourth ventricle (Stilling). 



The SIXTH NERVE ( 6 ), abducent nerve of the eyeball, springs from the 



pyramidal body close to the puns, and by a second band from the lower 

 part of the pons. 



Deep origin. The fibres of the nerve bend backwards, through the medulla 

 oblongata, to a nucleus in the floor of the fourth, ventricle, whose position is on 

 the outer part of the fasciculus teres, and behind the anterior fossa. See Anatomy 

 of the Fourth Ventricle (p. 217). 



The SEVENTH CRANIAL NERVE (Willis) ( 7 ) appears at the lower border 

 of the pons near the restiform body. It consists of two distinct trunks, 

 facial and auditory ; the former being the motor nerve of the face, and the 

 latter the special nerve of the organ of 'hearing. 



The facial nerve (portio dura, seventh nerve, Sommerring) is firm "and 



1 Untersuchungen uber den Bau des Hirnknotens. 



