606 CRANIAL NERVES. 



The middle, or gray root, arises from a papilla of gray matter (caruncula mam- 

 millaris), 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 of a somewhat prismoid form on section. 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 plexifonn 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 fibrillae, 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 dis- 

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



together at the commissure; and from the 

 Fig. 337.-The Optic Nerves and Optic back of the coram i ssure , they may be traced 



to the brain, under the name of the optic 

 tracts. 



The optic tract, at its connection with the 

 brain, is divided into two bands which arise 

 from 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, desti- 

 tute 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 con- 

 nected with the tasnia sernicircularis, and the 



anterior termination of tlTe gyrus fornicatus. It finally joins with the nerve of 



the opposite side to form the optic commissure. 



