OLFACTORY NERVE 633 



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 Sylvius, to the temporo-sphenoidal lobe. The internal white 

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

 most posterior portion of the frontal lobe. The middle, or gray root 

 arises from a little eminence of gray matter situated on the posterior 

 and inner portion of the inferior surface of the frontal lobe. 



The deep origin of the roots of the olfactory nerves is still a matter 

 of discussion. The external root passes through the gray substance of 

 the island of Reil, to a gray nucleus in the temporo-sphenoidal lobe, in 

 front of the pes hippocampi. The fibres of the middle root have not 

 been traced farther than the gray eminence from which they arise. The 

 fibres of the internal root probably are con- 

 nected with the fibres of the gyrus fornicatus. 

 The three roots converge to form a single 

 cord at the inner boundary of the fissure of 

 Sylvius. This cord passes forward and slightly 

 inward, in a deep groove between two convo- 

 lutions on the under surface of the frontal lobe 

 covered by the arachnoid membrane, to the k 



ethmoid bone. This portion of the nerve is ?^^.- Vertical section of 



the olfactory membrane of an exe- 

 SOft and friable. It is Composed of both white cuted criminal, x 250 (Zimmer- 



and gray matter, the proportions being about mann and Sobotta) ' 

 two-thirds of the former to one-third of the fibr f s ' ; %*% d. 01 ^ 

 latter. The gray substance, derived from the 



gray root, is at the upper portion of the nerve, the white substance 

 occupying the inferior and the lateral portions. 



By the side of the crista galli of the ethmoid bone, the nerve-trunk 

 expands into an oblong ganglion called the olfactory bulb. This is 

 grayish in color, very soft, and contains the ordinary ganglionic ele- 

 ments. From the olfactory bulb, fifteen to eighteen nervous filaments 

 are given off, which pass through foramina in the cribriform plate 

 of the ethmoid bone. These filaments are composed entirely of nerve- 

 fibres, and are quite resisting, owing to fibrous elements prolonged from 

 the dura mater. It is strictly correct, perhaps, to regard these as the 

 true olfactory nerves, the cord leading from the olfactory bulb to 

 the cerebrum being properly a commissure. Having passed through 

 the cribriform plate, the olfactory nerves are distributed to the olfactory 

 membrane, in three groups : an inner group, distributed to the mucous 

 membrane of the upper third of the septum ; a middle group, to the 

 upper portion of the nasal fossae ; and an outer group, to the mucous 



