728 DISSECTION OF THE BRAIN. 



inch across, and lies on the olivary eminence of the sphenoid bone, 

 situation, within the circle of Willis. It is placed in front of the tuber cine- I 

 reum ; and passing beneath it (in this position of the brain) is the 

 thin lamina cinerea. 



arrangement In the commissure each tract is resolved into three sets of fibres 

 of fibres. wifa the following arrangement : The outer fibres, few in number, 

 are continued straight to the temporal side of the eyeball of the 

 same side. The middle, the most numerous, decussate with the 

 corresponding fibres of the other tract, those of the right tract 

 being continued into the inner part of the left nerve and passing to 

 the nasal portion of the opposite eye, and vice versa. The most internal 

 fibres are continued across the back of the commissure into the tract 

 of the other side back to the brain without entering the eye, and 

 are not visual fibres. 



Trunk of The optic nerve extends from the commissure to the eyeball, and 



is aboMt one inch and a half in length. It leaves the skull by the 

 optic foramen, where it receives its sheaths from the dura mater and 

 arachnoid and crosses the orbit to end in the retina. 



Origin of the The THIRD or OCULOMOTOR NERVE ( :i ) is round and firm, and is 

 ve ' attached by a series of filaments along an oblique groove on the 

 inner side of the crus cerebri, near the posterior perforated space, 

 and close in front of the pons Varolii. 



deep in crus Deep origin. The fibres of the nerve traverse the crus in their course 

 cerebri. from a nuclear origin in the grey substance in the floor of the aqueduct of 

 Sylvius beneath the anterior corpus quadrigemiuum.* 



Fourth The FOURTH or TROCHLEAR NERVE ( 4 ) cannot be followed back- 



cerebeiium wards at P resent to its origin. It is the smallest of the cranial 



' nerves, and emerges on the upper surface of the crus behind the 



posterior corpus quadrigeininnm through the valve of Vieussens 



(fig. 277, 4 , p. 765). The nerve appears at the base between the 



cerebrum and cerebellum on the side of the crus cerebri, and is 



directed forwards to enter an aperture in the free edge of the tentorium 



cerebelli near the posterior clinoid process. 



nucleus in Deep origin. In the valve of Vieussens the nerve crosses to the opposite 

 floor of gjde^ decussating with its fellow, and then arches round the aqueduct of 

 aqueduct Sylvius to reach its nucleus in the floor of that canal, immediately behind the 

 nucleus of the third nerve. 



Fifth nerve The FIFTH or TRIGEMINAL ( 5 ) is the largest of the cranial nerves, 

 roots7 anc ^ C01is i sts of two roots, ganglionic or sensory, and aganglionic or 



motor, which are separate to beyond the ganglion. 



both issuing The nerve emerges from the side of the pons Varolii, nearer the 

 from pons. U pp er t j, an t jj e lower border. The small or aganglionic root is the 

 higher, and is separated from the large root by one or two of the 

 transverse bundles of the pons. Both roots pass outwards through 

 an aperture in the dura mater, above the petrous part of the 

 temporal bone into the cavum Meckelii, as already described, p. 516. 



* The position of the nuclei of this and the following nerves is roughly 

 shown on fig. 287, on p. 783. 



