536 CRANIAL NERVES. 



The middle or gray root arises from a papilla of gray matter, the caruncula 

 mamrnillaris, imbedded in the anterior lobe. This root is prolonged into the 

 nerve from the adjacent part of the brain, and contains white fibres in its interior, 

 which are connected with the corpus striatum. 



The internal or short root is composed of white fibres, which arise from the 

 inner and back part of the anterior lobe, being connected, according to Foville, 

 with the longitudinal fibres of the gyrus fornicatus. 



These three roots unite, and form a flat band, narrower in the middle than at 

 either extremity, and its section of a somewhat prismoid form. It is soft in texture, 

 and contains a considerable amount of gray matter in its substance. As it passes 

 forwards, it is contained in a deep sulcus, between two convolutions, lying on the 

 under surface of the anterior lobe, on either side of the longitudinal fissure, and 

 is retained in position by the arachnoid membrane which covers it. On reaching 

 the cribriform plate of the ethmoid bone, it expands into an oblong mass of grayish- 

 white substance, the olfactory bulb. From the under part of this bulb are given 

 off numerous filaments, about twenty in number, which pass through the cribriform 

 foramina, and are distributed to the mucous membrane of the nose. Each filament 

 is surrounded by a tubular prolongation from the dura mater and pia mater; the 

 former being lost on the periosteum lining the nose ; the latter, in the neurilemma 

 of the nerve. The filaments, as they enter the nares, are divisible into three 

 groups : an inner group, larger than those on the outer wall, spread out over the 

 upper third of the septum ; a middle set, confined to the roof of the nose ; and 

 an outer set, which are distributed over the superior and middle turbinated bones, 

 and the surface of the ethmoid in front of them. As the filaments descend, they 

 unite in a plexiform network, and become gradually lost in the lining membrane. 

 Their mode of termination is unknown. 



The olfactory differs in structure from other nerves, in containing gray matter 

 in its interior, and being soft and pulpy in structure. Its filaments are deficient 

 in the white substance of Schwann, are not divisible into fibrillas, and resemble 

 the gelatinous fibres in being nucleated, and of a finely-granular texture. 



OPTIC NERVE. 



The SECOND or OPTIC NERVE, the special nerve of the sense of sight, is 

 distributed exclusively to the eyeball. The nerves of opposite sides are connected 

 together at the commissure ; and from the back of the commissure, they may be 



traced to the brain, under the name of the 

 Fig. 273. The Optic Nerves and Optic Tracts. o ~fa tracts 



The optic tract, at its connection with 

 the brain, divides into two bands which 

 are continued into the optic thalami, the 

 corpora geniculata, and the corpora 

 quadrigemina. The fibres of origin from 

 the thalarnus may be traced partly from 

 its surface, and partly from its interior. 

 From this origin, the tract winds obliquely 

 across the under surface of the crus cerebri, 

 in the form of a flattened band, destitute 

 of neurilemma, and is attached to the crus 

 by its anterior margin. It now assumes a 

 cylindrical form, and, as it passes forwards, 

 is connected with the tuber cinereum, and 

 lamina cinerea, from both of which it 

 receives fibres. According to Foville, it 

 is also connected with the tsenia semi- 

 circularis, and the anterior termination of the gyrus fornicatus. It finally joins 

 with the nerve of the opposite side, to form the optic commissure. 



