THE CRANIAL NERVES. 



575 



Sylvius and come into relation with nerve-cells in the inferior extremity'of 

 the gyrus hippocampus and the gyrus uncinatus. The mesial fibers pass 

 inward and come into relation with nerve-cells in the pre-callosal part at 

 least of the gyrus fornicatus. The fibers thus far considered are undoubt- 

 edly true olfactory fibers, pursuing a centripetal direction, carrying nerve 

 impulses from the olfactory cells to the cerebrum (Fig. 264). 



Histologic and embryologic methods of research have shown that some 

 of the fibers in the olfactory tract are centrifugal in function. They originate 

 in the olfactory cortical areas, pass toward the periphery as far as the an- 

 terior commissure, where they 

 cross to become the dorsal 

 root, enter the olfactory tract, 

 and finally terminate in the 

 bulb. This tract serves to 

 connect the cortex with the 

 bulb of the opposite side, and 

 carries impulses from the cor- 

 tex to the bulb. The two op- 

 posite cerebral olfactory areas 

 are also united by commissural 

 fibers which decussate at the 

 anterior commissure. 



Function. The function 

 of the olfactory system in its 

 entirety is the transmission of 

 nerve impulses from its origin 

 in the olfactory region of the 

 nose to the cerebral cortex, 

 where they evoke sensations 

 of odor. The stimulus to its 

 excitation is the impact and 

 chemic action of gaseous or 

 volatile organic matter on the 

 dendrites of the olfactory cells. 

 The sensitiveness of the olfac- 

 tory end-organ to the action 

 of many substances is remark- 

 able, responding, for example, to the TTimy or f a gram of oil of roses and to 

 the 2T6~ro o"o f a g ram of mercaptan. 



Division or destruction of the olfactory path at any point is followed 

 by an abolition of the sense of smell on the corresponding side. Destructive 

 lesions of the hippocampal and uncinate gyri are followed by similar results. 



SECOND NERVE. THE OPTIC. 



The second cranial nerve, the optic, consists of centrally coursing axons 

 of neurons which connect the essential part of the organ of vision, the retina, 

 with sensory end-nuclei or ganglia situated at the base of the cerebrum. 



Origin. The axons which constitute the optic nerve have their origin 

 in the ganglion cells in the anterior part of the retina. Through their 



CH. 



T.o. 



FIG. 264. OLFACTORY LOBE OF THE HUMAN BRAIN. 

 Bu. Olfactory bulb. T. Tract. Tr.o. Trigone. R. 

 Rostrum of corpus callosum. p. Peduncle of corpus 

 callosum, passing into G. s., gyrus subcallosus (diagonal 

 tract, Broca). Br. Broca's area. P.p. Fissura prima. 

 F.s. Fissura serotina. C.a. Position of anterior com- 

 missure. L.t. Lamina terminalis. Ch. Optic chiasma. 

 T.o. Optic tract, p. ol}. Posterior olfactory lobule (or 

 anterior perforated space), m.r. Mesial root. l.r. 

 Lateral root of tract. (His.) After Quain.) 



