THE CRANIAL NERVES 277 



twenty in number and pass through the foramina in the cribri- 

 form plate to be distributed to the mucous membrane (Schnei- 

 derian) 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 inexcit- 

 able. 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 recognized as 

 odors only. Removal of the olfactory bulb in a dog is evidently 

 followed by a loss of the sense so characteristic 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 commissure. The optic com- 

 missure occupies the optic groove on the superior surface 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 body, 

 from the pulvinar of the optic thalamus and from the superior 

 corpus quadrigeminum; the internal comes from the internal 

 geniculate body. These two, uniting, cross the crusta obliquely 

 to reach the optic commissure, or chiasm. In the commissure 

 the fibers from the inner margin of each optic tract pass to the 

 other side of the brain, and may be called commissural fibers 

 between the internal geniculate bodies. Some fibers anteriorly 

 connect the two optic nerves with each other and are not prop- 

 erly part of the chiasm, but connect the two retinae. The outer 

 fibers of each tract pass to the nerve of the same side, while 

 the central fibers decussate in the commissure with similar fibers 



