358 THE PERIPHERAL NERVOUS SYSTEM 



The ganglionated n. terminalis courses in close association with the olfactory 

 nerve. Its fibers end in the epithelium of the vomero-nasal organ and of the 

 nose. Although evidently a distinct nerve its significance is obscure. 



2. The Optic Nerve is formed by fibers which take their origin from neuro- 

 blasts in the nervous layer of the retina. The retina is differentiated from the 

 wall of the fore-brain and remains attached to it by the optic stalk (Fig. 343), 

 hence the optic nerve is not a true peripheral nerve, but belongs to the central 

 system of tracts. The neuroblasts from which the optic nerve fibers develop 

 constitute the ganglion cell layer of the. retina (Fig. 381). During the sixth and 

 seventh weeks these cells give rise to central processes which form a nerve fiber 

 layer on the inner side of the retina. The optic fibers converge to the optic stalk 

 and grow through its wall back to the brain. The cells of the optic stalk are 

 converted into a neuroglia framework and the cavity is obliterated. In the floor 

 of the fore-brain, at the boundary between telencephalon and diencephalon, 

 the fibers from the median half of each retina at about the end of the second 

 month cross to the opposite side, and this decussation constitutes the optic chiasma 

 (from Greek letter X or "chi"). The crossed and uncrossed fibers constitute the 

 optic tract which rounds the cerebral peduncles laterally and dorsally (Fig. 354). 

 Eventually the optic fibers end in the lateral geniculate body, thalamus, and 

 superior colliculus. 



Efferent fibers, terminating in the inner reticular layer of the retina, are also 

 present. In certain fishes where their function has been studied these fibers 

 resemble visceral efferent components (Arey, Jour. Comp. Neurol., vol. 26, 1916). 



8. The Auditory Nerve, or N. Acusticus, is formed by fibers which grow 

 from the cells of the acoustic ganglion. The origin of these cells is unknown, 

 though they appear in 4 mm. embryos just cranial to the otic vesicle (Fig. 358). 

 The cells become bipolar, central processes uniting the ganglion to the tuberculum 

 acusticum of the myelencephalon and peripheral fibers connecting it with the 

 wall of the otocyst. The acoustic'ganghon is differentiated into the vestibular and 

 spiral ganglia (Fig. 362). Its development has been studied by Streeter (Amer. 

 Jour. Anat., vol. 6, 1907). The ganglion elongates and is subdivided into superior 

 and inferior portions in 7 mm. embryos. The superior part suppUes nerves to 

 the utriculus and to the ampullae of the anterior and lateral semicircular canals. 

 It forms part of the vestibular ganglion of the adult. Part of the inferior portion 

 supplies nerves to the sacculus and to the ampulla of the posterior semicircular 

 canal, and this portion, together with the pars superior, constitutes the vestibular 

 ganglion. The greater part of the pars inferior is, however, differentiated into the 



