STRUCTURE OF THE VERTEBRATES 307 



1. Retina. The retina consists of three rather distinct layers 

 of cells: (1) an outer receptor layer (in contact with the pig- 

 mented layer) ; (2) a middle group of association neurones with 

 short processes; and (3) an inner layer (next the cavity) of 

 association cells with long axones which pass to the brain. Due 

 to their shape the sensory cells are known as rod and cone cells. 

 These are partially embedded in the pigmented layer. Stimuli 

 are received by the rods and cones, transferred to the short 

 association cells, and then passed to the inner association layer. 

 Thus, a ray of light entering the eye passes through two layers 

 of retinal cells before reaching the receptors. The axones of 

 the inner layer of association neurones converge toward a cen- 

 tral point where they enter the optic stalk. Near the exit of the 

 optic nerve the axones are close packed and the area occupied 

 by the nerve is not sensory. This is the blind spot of the eye. 

 The area surrounding the blind spot is packed with sensory 

 cells, and this region (the macula lutea) is extremely sensitive. 

 Between the nerve cells of the retina are ectodermal supporting 

 cells, resembling the similar neuroglia of the brain. 



The pigmented layer, which lies in contact with the meso- 

 dermal covering, is very thin and becomes an integral part of 

 the retina. The cells expand or shrink, assisting other eye 

 structures in protecting the sensory cells from too intense light. 



2. Choroid layer. The choroid layer is the inner portion of 

 the optic capsule, lying in contact with the pigmented layer of 

 the retina. It corresponds functionally with the pia mater of 

 the brain. It is highly vascular and is the major nutrient source 

 of the eye. The choroid is in close contact with the sclerotic 

 layer throughout most of its area, but on the distal surface the 

 two separate. The outer sheath is continued over the eye as the 

 cornea; the choroid forms (1) the circular iris, and (2) the 

 ciliary process. The latter arises at the point where the iris 

 separates from the cornea, and is the muscular base to which 

 the lens is attached. The ciliary muscles move the eye forward 

 and backward and slightly adjusts its shape. The iris continues 

 over the lens as a curtain with a central opening, the pupil 

 of the eye. The change in pupillary size is caused by the con- 

 striction or dilation of the iris by its sphincter muscles. The 

 iris is normally pigmented, giving color to the eye. The dark 



