356 THE PERIPHERAL NERVOUS SYSTEM 



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. 325). 

 The neuroblasts from which the optic nerve fibers develop constitute the gang- 

 lion cell layer of the retina (Fig. 362). 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 neu- 

 roglia framework and the cavity is obliterated. In the floor of the fore-brain 

 at a point which forms the boundary line between telencephalon and dienceph- 

 alon, the fibers from the median half of each retina cross to the opposite side 

 (decussate), and this crossing constitutes the optic chiasma (from Greek letter X or 

 "chi"). The decussation of the optic fibers takes place about the end of the sec- 

 ond month. The crossed and uncrossed fibers constitute the optic tract which 

 rounds the cerebral peduncles laterally and dorsally (Fig. 336). Eventually, the 

 optic fibers end in the lateral geniculate body, thalamus and superior colliculus. 



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

 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. 340). 

 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 ganglion is differentiated into the vestibular and 

 spiral ganglia (Fig. 343) . Its development has been studied by Streeter (Amer. 

 Jour. Anat., vol. 6). 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 anterior and lateral semicircular canals. It forms part 

 of the vestibular ganglion of the adult. The inferior portion supplies nerves to the 

 sacculus and to the ampulla of the posterior semicircular canal and this portion 

 of it with the pars superior constitutes the vestibular ganglion. The greater 

 part of the pars inferior is,, however, differentiated into the spiral ganglion, the 

 peripheral fibers of which innervate the hair cells of the spiral organ (of Corti) 

 in the cochlea. The spiral ganglion appears in 9 mm. embryos and conforms to 

 the spiral turns of the cochlea, hence its name. Its central nerve fibers form the 

 cochlear division of the acoustic nerve. This is distinctly separated from the 

 central fibers of the vestibular ganglion which constitute the vestibular division 

 of the acoustic nerve, the fibers of which are not auditory in function. The pars 

 inferior of the vestibular ganglion becomes closely connected with the n. coch- 





