NEUROLOGY 267 



Fifth, a layer of epithelial cells. 



The peripheral fibers and the nerve cell layers near the hemi- 

 spheres disappear so that the basic substance of the trabecula 

 with the hemispheres form the entire lobe mass. On the lower 

 surface of the hemispheres there is a long bundle of nerve fibers 

 which enter into the substance of the olfactory lobes and there 

 disappears. These fibers are medullated. Non-medullated fibers 

 enter into the makeup of the olfactory trunk. 



The original center of the olfactory nerve is not in the hemisphere, 

 but in the same location as the optic nerve. This nerve trunk con- 

 sists of very fine non-meduUated fibbers. The nerves of smell are 

 therefore not peripheraUstic nerves. The nerve fibers are distributed 

 to the mucous surface of the turbinated bones. Toward the front 

 they form an expanded prolongation. 



The olfactory nerve, as it emerges from the cone-like anterior tip 

 of the cerebrum, is of considerable thickness and extends along with 

 the median dorsal artery of that region above and to the inside of 

 the orbit, under the thin bony structure. Before its termination 

 into the posterior turbinated bones, it is crossed by the superior 

 maxiUary division of the fifth pair of cranial nerves. It extends as 

 far as the pituitary membrane of the turbinate bones upon which its 

 filaments are distributed radially. 



The Opticus. — The nervus opticus (Fig. 75, No. C, 14) the second 

 cranial, is the nerve of sight. The optic lobe, or tuberculum 

 bigeminum lies at the base of the brain on each side of the optic 

 tract. 



From the optic lobes the two trunks pass forward along the under 

 surface of the cerebri, forming the optic chiasm at the hypophysis 

 (Fig. 75, No. C, 15), at which point the nerve fibers originating from 

 the right side pass to the left, and vice versa. From the chiasm the 

 true optic nerves pass forward to the posterior surface of the eyeball. 

 They are composed of a bundle of very fine, marrow-Uke nerves. 

 These fibers enter into the gangUonic cells of the retina. By re- 

 moval of the finely adherent neurilemma, the optic nerve is seen to 

 be composed of parallel, longitudinal lamellse, the margins of which 

 are mostly free on one side. 



The Motor Oculi. — The motor ocuU (Fig. 62, No. C, 10), the 

 third cranial nerve is a motor nerve. It originates close to the 

 base of the brain behind the position of the hippocampus of mam- 

 mals on the inner side of the crus cerebri, and also on the inner 



