370 THE NERVE SYSTEM 



of a slender process of brain substance, called the 

 olfactory tract. The olfactory nerves differ in struc- 

 ture from the other nerves, containing only amyelinic 

 fibers. 



THE OPTIC .NERVES (SECOND) . The fibers of the 

 optic nerves, the special nerve of the sense of sight, 

 are situated in the retina; they start as the central 

 processes of the ganglion cells which converge and 

 pierce the choroid and sclera as a cylindric cord. The 

 point of emergence is situated a little internal to the 

 posterior pole of the globe. Passing through the orbital 

 fat, in an inward and backward direction, it passes 

 through the optic foramen to end in the optic chiasm 

 or commissure. The optic chiasm is somewhat quadri- 

 lateral in shape, rests on the olivary eminence and the 

 diaphragma sellse, being bounded above by the lamina 

 terminalis; behind, by the tuber cinereum, on either 

 side by the anterior perforated substance. Within 

 the chiasm the fibers cross as follows: Those from 

 the nasal side of the left and right halves of the retina 

 cross in the centre, to the opposite optic tract; those 

 from the temporal side of the right and left eyes pass 

 backward without crossing, to end in the optic tract 

 of the same side. 



The optic tract passes back to areas of the brain 

 where the nerve impulses are conveyed to the cuneus 

 (the area for the sense of sight in the cortex) by means 

 of another pathway, the optic radiation. By connections 

 with other nerve centres in the brain the optic tract 

 communicates w r ith the origin of the nerves which 

 influence the muscles that control the movements of 

 the eye-ball. 



THE OCULOMOTOR (THIRD). The oculomotor arises 

 superficially from the crus anterior to the pons, its 

 deep origin being a gray nucleus in the floor of the 

 aqueduct of Sylvius. It runs to the outer side of the 

 posterior clinoid process of the sphenoid bone, enters 



