3 8 2 



THE PERIPHERAL NERVOUS SYSTEM 



(Fig. 377). These correspond to the cellular layer (ependymal and mantle 

 zones) and marginal layer of the neural tube. In fetuses of 65 mm. (C R) 

 the retina shows three layers, large ganglion cells having migrated in 

 from the outer cellular layer of rods and cones (Fig. 380). In a fetus of 

 the seventh month all the layers of the adult retina may be recognized 

 (Fig. 381). As in the wall of the neural tube, there are differentiated in 

 the retina supporting tissue and nervous tissue. The supporting elements, 

 or fibers of Muller, resemble ependymal cells and are radially arranged 



(Figs. 380 and 381). Their 

 terminations form internal 

 and external limiting mem- 

 branes. 



The neuroblasts of the 

 retina differentiate into an 

 outer layer of rod and cone 

 cells, the visual cells of the 

 retina, which are at first 

 unipolar (Fig. 381). In- 

 ternal to this layer are 

 layers of bipolar and mul- 

 tipolar cells. The inner 

 layer of multipolar cells 

 constitutes the ganglion 

 cell layer. Axons from 



Pigment layer 

 Rods and Cones 



Outer nuclear layer 



Outer reticidar layer 

 Inner nuclear layer 



Inner reticular layer 

 Ganglion cell layer 



Nerve fiber layer 

 Fibers of Midler 



'Internal limiting membrane 

 FIG. 381. Section through the pars optica of the retina 

 from a seven months' fetus. X 440. 



these cells form the inner 

 nerve fiber layer of optic 

 fibers. These converge to 

 the optic stalk, and, in 

 embryos of 15 mm. grow 

 back in its wall to the brain. The cells of the optic stalk are converted 

 into neuroglia supporting tissue and the cavity in the stalk is gradually 

 obliterated. The optic stalk is thus transformed into the optic nerve (cf . 

 P- 358). 



The Sclerotic and Chorioid Layers, and their Derivatives. After the 

 mesenchyme grows in between the ectoderm and the lens (Fig. 377), the 

 lens and optic cup are surrounded by a condensed layer of mesenchymal 

 tissue, which gives rise to the supporting and" vascular layers of the eye- 

 ball. By condensation and differentiation of its outer layers, a dense layer 

 of white fibrous tissue is developed, which forms the sclera. This cor- 

 responds to the dura mater of the brain. In the mesenchyme of 25 mm. 

 embryos, a cavity appears distally which separates the condensed layer of 

 mesenchyme, continuous with the sclerotic, from the vascular capsule of 



