358 THE PERIPHERAL NERVOUS SYSTEM 



epithelium of the vestigial vomero-nasal organ (of Jacobson) also end in 

 the olfactory bulb. 



When the ethmoidal bone of the cranium is developed, its cartilage, as 

 the cribriform plate, forms around the strands of the olfactory nerve. 



The ganglionated n. terminates courses in close association with the 

 olfactory nerve. Its unmyelinated 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 

 neuroblasts in the nervous layer of the retina. The retina is differentiated 

 from an evagination of the wall of the fore-brain (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 frame- 

 work 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, 1916). 



8. The Acoustic Nerve 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 tuber- 

 culum acusticum of the myelencephalon and peripheral fibers connecting it 

 with the wall of the otocyst. 



The acoustic ganglion is differentiated into the vestibular and spiral 

 ganglia (Fig. 362) . The ganglion elongates and is subdivided into superior 

 and inferior portions in 7 mm. embryos. The superior part supplies 

 nerves to the utriculus and to the ampullae of the anterior and lateral semi- 

 circular canals. 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 entire pars superior, constitutes the vestibular ganglion. 



