3 8o 



THE PERIPHERAL NERVOUS SYSTEM 



of optic fibers. These converge to the optic stalk and grow back in its wall to 

 the brain. The cells of the optic stalk are converted into neuroglia supporting 

 tissue and the cavity of the stalk is gradually obliterated. The optic stalk is 

 thus transformed into the optic nerve (see p. 356). 



The Sclerotic and Chorioid Layers, and their Derivatives. — After the mes- 

 enchyme grows in between the ectoderm and the lens (Fig. 358) the lens and op- 

 tic cup are surrounded by a condensed layer of mesenchymal tissue, which gives 

 rise to the supporting and vascular layers of the eyeball. By condensation and 



differentiation of its outer layers, a 



P^ifPgilE^^S^S' — Pigment layer 



flods and Cone 



Outer nuclear layer 



_ji»MSl^P$S Pods and Cones 



dense layer of white fibrous tissue is 

 developed, which forms the sclerotic 

 layer. This corresponds to the dura 

 mater of the brain. In the mesen- 

 chyme of 25 mm. embryos a cavity 

 appears distally, which separates 

 the condensed layer of mesenchyme 

 continuous with the sclerotic from 

 the vascular capsule of the lens (Fig. 

 360). This cavity is the anterior 

 chamber of the eye and separates 

 the anlage of the cornea from the 

 lens capsule. An inner layer of 

 mesenchyme, between the anlage 

 of the sclerotic and the pigment 

 layer of the retina, becomes highly 

 vascular during the sixth month. 

 Its cells become stellate in form and 

 pigmented so that the tissue is loose 

 and reticulate. This vascular tissue constitutes the chorioid layer in which course 

 the chief vessels of the eye. The chorioid layer corresponds to the pia mater of 

 the brain. Distal to the ora serrata of the retinal layer the chorioid is differen- 

 tiated: (1) Into the vascular folds of the ciliary bodies; (2) into the smooth 

 fibers of the ciliary muscle; (3) into the stroma of the iris. The proximal pig- 

 mented layers of the iris are derived from the pars iridis retinae and from a cor- 

 responding zone of the pigment layer. Of these the pigment layer cells give 

 rise to the sphincter and dilator muscles of the iris. These smooth muscle fibers 

 are thus of ectodermal origin. 



jg Outer reticularlayer 



Inner nuclear layer 



nner reticular layer 

 Ganglion cell layer 



Nerve fiber layer 

 Tibers of Mueller 



Internal limiting 

 membrant 



Fig. 362. — Section through the pars optica of the 

 retina from a seven months' fetus. X 440. 



