THE VISUAL CELL-LAYER 45 



the ciliary body, as the innemiost of the two layers of the ciliary epi- 

 thelium, to the ora terminalis. At this point the simple epithelium sud- 

 denly becomes stratified and complex to form the sensory retina. 



Travelling thus forward to the pupil in the pigment epithelium and 

 backward again into the sensory retina proper, we are easily able to see 

 that the entire retinal coat of the eye reaches to the pupil margin and 

 forms a two-layered cup. The two major layers — pigment epithelium 

 and retina proper — develop directly from the two layers of the embry- 

 onic optic cup, which arises as a bubble of tissue on the side of the 

 brain, becoming constricted off therefrom and deeply indented on the 

 side toward the skin. This indentation gives the vesicle an outer and an 

 inner layer and an opening, aimed toward the surface of the head, into 

 which the lens is received after its separation from the skin (see Fig. 38, 

 p. 106). Thereafter the opening becomes (relatively) smaller, and per- 

 sists as the pupil. 



The Visual-Cell Ltayer— Standing on the external surface of the 

 retina proper, and constituting its receptive layer, are the rods and cones 

 (Fig. 19). These elongated cells thus point away from the light, which 

 must pass through the remainder of the retina to reach them (hence the 

 complete transparency of this tissue as contrasted with the brain, which 

 has a similar histological organization). Their tips are pressed against 

 the pigment cells or are even buried in deep indentations in them, or 

 between their processes when such are present. The processes in turn 

 may reach nearly to the bases of the rods and cones so that they are 

 deeply interdigitated with the latter. Though there is seldom a conti- 

 nuity of substance, the dovetailing of the sets of processes and visual 

 cells is so intimate and firm that one or the other is often torn in two 

 if the retina and chorioid are forcibly separated. In other cases the 

 absence of all pigment-cell processes may make a separation very easy, 

 and only the optic nerve, the fusion of the two layers of the optic cup 

 at the ora terminalis, and the pressure of the vitreous then hold the 

 retina firmly and smoothly in place. 



At the level of the bases of the rods and cones the retina has its ex- 

 ternal limiting membrane (briefly, the 'limitans') which may be likened 

 to a piece of wire screening through each hole of which a rod or cone 

 projects. The visual cells are a tight fit for the holes and are thus kept 

 perpendicular to the membrane and prevented from getting out of line 

 by any sliding lengthwise past each other. In some retinas, delicate hair- 

 like processes from the outer surface of the membrane itself form so- 



