ENCEPHALIC NERVES. 449 



cells of -rsiotf to ^Vir of an inch, many with branched fibres, are 

 found among the nerve-fibres. The latter very closely resemble 

 the embryonic nerve-elements, and are probably derived from the 

 olfactory bulb, and not from the cerebrum itself. It is doubtful 

 how they terminate. The surface over which they are distributed, 

 called the " olfactory region," extends only f to 1 inch below the 

 lamina cribrosa of the ethmoid bone (Fig. 290) ; and on this surface 

 alone the epithelium of the mucous membrane is not ciliated. (Fig. 

 291.) 



2. The second pair of encephalic nerves is the nerve of vision. 

 The optic nerve has its tract composed of dark-bordered fibres g^^ 

 of an inch in diameter. In the chiasma, some of the fibres pass to 

 the retina of the same side, and some to faiat of the other side ; 

 while others still, form a loop in the posterior part of the chiasma, 

 and thus connect the origins of these nerves ; and a fourth set are 

 looped in a similar manner anteriorly to connect the two retinae. 

 (Fig. 293.) The fibres are much disposed to become varicose, but 



Fig. 293. 



Course of fibres in the optic cbiasma, as exhibited by tearing off the superficial bundles from a 

 specimen hardened in spirits, a. Anterior fibres commissural between the two retinae, p. Posterior 

 fibres commissural between the thalami. a', p'. Diagram of the preceding. 



the nerve-cells among them, mentioned by Hassall, have not been 

 found by Kolliker. They form polygonal bundles ^ to T 7 of 

 an inch in diameter, surrounded by a perineurium of the usual 

 kind, and which is replaced by the sclerotica on their reaching the 

 eyeball. The fibres undergo no change till after entering within 

 the sclerotica and forming the slight elevation (colliculus nervi 

 optici), on the retina opposite its point of entrance. But from that 

 point onwards, the fibres become perfectly clear, yellowish or gray- 

 ish, and transparent, like the finest fibres in the central organs, and 

 mostly from g^J^o to T^TT^ of an inch, though some are T2 <nr to 

 gfiW of an inch. They, however, have no nuclei in their course, 

 and are inclined to varicosities like the minutest fibres of the cere- 

 brum. (Kolliker.) (Figs. 294 and 298, 3.) 

 29 



