THE CRANIAL NERVES 2/7 



gyrus fornicat.us; the middle from the under surface of the 

 frontal lobe anterior to the anterior perforated space; the 

 external from the temporo-sphenoidal lobe. These three 

 roots unite to pass forward underneath the frontal lobe near 

 the longitudinal fissure as the olfactory tract. The deep 

 origin is unsettled. 



Course and Distribution. Reaching the upper surface of 

 the cribriform plate of the ethmoid, the olfactory tract ex- 

 pands into the olfactory bulb, from the under surface of 

 which are given off the special nerve fibers of the sense of 

 smell. They are about twenty in number and pass through 

 the foramina in the cribriform plate to be distributed to the 

 mucous membrane (Schneiderian) of the nose in three sets 

 an inner to the upper third of the septum, a middle to the 

 roof of the nares, and an outer to the superior and middle 

 turbinated bones and the ethmoid in front of them. The 

 fibers are non-medullated. 



Function. The olfactory nerves are insensible and inex- 

 citable. They are concerned with the sense of smell alone 

 and their integrity is necessary to the preservation of that 

 sense. They convey to the brain impressions which are rec- 

 ognized as odors only. Removal of the olfactory bulb in a 

 dog is evidently followed by a loss of the sense so charac- 

 teristic of the animal. Furthermore, the olfactory bulbs in 

 lower animals are shown to be developed in proportion to the 

 acuteness of the sense of smell. 



Second Nerve (Optic). 



Origin. This is the nerve of sight. Its apparent origin is 

 from the anterior part of the optic cgmmissure. The optic 

 commissure occupies the optic groove 'on the superior sur- 

 face of the sphenoid. It represents the union of the two 

 optic tracts each of which, traced backward, is found to 

 divide into two bands ; the external takes its origin from the 

 external geniculate b.ody, from the pulvinar of the optic thai- 



