OLFACTORY NERVES. 27 







secrete a fluid wliicli keeps the surface moist, a condition es- 

 sential to the accurate perception of odorous impressions. 



Physiological Anatomy of the Olfactory Nerves. The 

 apparent origin of the olfactory nerve is by three roots, from 

 the inferior and internal portion of the anterior lobe of the 

 cerebrum, in front of the anterior perforated space. The three 

 roots are an external and an internal white root, and a middle 

 root composed of gray matter. The external white root is 

 long and delicate, passing outward across the fissure of Syl- 

 vius to the middle lobe of the cerebrum. The internal white 

 root is thicker and shorter than the external root, and arises 

 from the most posterior portion of the anterior lobe. The 

 middle, or gray root, arises from a little eminence of gray 

 matter situated on the posterior and inner portion of the in- 

 ferior surface of the anterior lobe. 



The deep origin of these three roots is still a matter of 

 discussion. The external root is stated by various anatomists 

 to originate from the corpus striatum, the optic thalamus, the 

 anterior commissure, and the island of Reil ; but researches 

 upon this point have been by no means satisfactory. The 

 same uncertainty exists with regard to the deep origin of the 

 internal white root and the gray root. 



The three roots of the olfactory converge to form a sin- 

 gle nervous cord at the inner boundary of the fissure of Syl- 

 vius. This passes forward and slightly inward in a deep 

 groove between two convolutions on the under surface of the 

 anterior lobe, covered by the arachnoid membrane, to the 

 ethmoid bone. This portion of the nerve is exceedingly soft 

 and friable. It is composed of both white and gray matter, 

 the proportions being about two-thirds of the former to one- 

 third of the latter. The gray substance, derived from the 

 gray root, is situated at the upper portion of the nerve, the 

 white substance occupying the inferior and lateral portions. 



By the side of the crista galli of the ethmoid bone, the 

 tferve-trunk expands into an oblong ganglion, called the olfac- 



