THE CRANIAL NERVES. 219 



The nerves of general sensibility have their origin in the ganglia 

 situated on their trunks. 



First Pair. Olfactory. 



The olfactory nerve is situated in the upper third of the nasal 

 fossa. It consists of from 20 to 30 branches. 



Origin. From neuro-epithelial cells situated among the epithelial 

 cells covering the mucous membrane. From these cells the nerve- 

 fibers pass upward through foramina in the cribriform plate of the 

 ethmoid bone and arborize around nerve-cells, in the olfactory bulb. 



The Olfactory Tract. The olfactory tract consists of both gray 

 and white fibers which pass from their origin in the bulb, to the 

 base of the cerebrum where it divides into three branches, viz., an 

 external white root, which passes across the fissure of Sylvius to the 

 middle lobe of the cerebrum ; an internal white root, which passes 

 also into the middle lobe ; a gray root, which is in relation with the 

 anterior lobe. The white fibers at least terminate around nerve-cells 

 in the gray matter of the pre-callosal part of the gyrus fornicatus, 

 the gyrus hippocampus and the gyrus uncinatus. 



Properties. The olfactory nerves do not give rise to either motor 

 or sensor phenomena when stimulated. When stimulated at their 

 periphery by odorous particles, nerve impulses are developed which, 

 when conducted to the brain, evoke the sensation of smell. Destruction 

 of the olfactory nerves, the bulb or tract, is followed by a loss of the 

 sense of smell. 



Function. Presides over the sense of smell. Conducts impulses 

 to the cerebrum which give rise to odorous sensations. 



Second Pair. Optic. 



Origin. The optic nerve arises from large nerve-cells in the 

 anterior part of the retina. From this origin the nerve-fibers turn 

 backward and converge to form a well-defined bundle (the optic 

 nerve) which passes out of the eyeball, through the orbit cavity 

 as far as the sella turcica. At this point there is a union and partial 

 decussation, in man at least, of the fibers, forming what is known 

 as the optic chiasm. From the posterior portion of the chiasm there 

 passes backward on either side a bundle of nerve-fibers, the optic 

 tract. Each tract contains nerve-fibers which come from the temporal 



