SIGHT. 



163 



the anterior chamber, and contains the aqueous fluid, while the 

 posterior is the vitreous humor. These structures fill the eyeball 

 and give it a tense feel to the touch. 



What are the coats of the eyeball ? 



(1) The sclera, or sclerotic coat, is external, and covers about 

 five-sixths of the globe. The cornea is continuous with it in front. 

 It is composed of tough white fibres arranged in intercommuni- 

 cating layers. (2) The choroid coat is very vascular, being com- 

 posed of a mesh of capillaries. There 

 is outside of this a layer of connective FIG. 48. 



tissue containing pigment-granules. 

 (3) The retina, which is the seat of 

 the end-organs of the optic nerve. 



Describe the retina. 



It consists of the nervous mechanism 

 of the eye lying in a loose connective 

 tissue, and beneath it a pigment-layer. 

 Without entering into the details of 

 its numerous layers, we may say that 

 the fibres of the optic nerve spread 

 out, divested of neurilemma, in this 

 membrane, and communicate with 

 ganglion-cells, which are abundant. 

 The fibres pass inward and terminate 

 in the layer of rods and cones which 

 form the end-organs of the nerve. 



Describe the rods and cones of the 



retina. 



They are closely packed at the sur- 

 face of the retina, the rods being the 

 more numerous in most situations. 

 The cones seem to be modifications 

 of the rods, and their office is essen- 

 tially similar. The rods (Fig. 48) are 

 straight cylindrical bodies of a trans- 

 parent substance, and are placed par- 

 allel to one another and perpendicular 

 to the surface of the eyeball. In length they are about five to seven 

 times the diameter of a red blood-corpuscle, and in diameter about 



Diagrammatic Section from the pos- 

 terior Portion of the Human Ret- 

 ina: 1, layer of rods and cones; 

 2, layer of nuclei (Schultze). 



