270 



HUMAN PHYSIOLOGY 



The mosaic arrangement of rods and cones lies in the external 

 layer of the retina (viewed from the centre of the 

 eyeball). To arrive at them, light coming from 

 the vitreous humor must first pass through all the 

 other layers of the retina. 



The outer layer of the retina in the macula lutea 

 contains only cones ; in the other portions it con- 

 tains cones and rods. In the central part of the 

 macula lutea, that is, in ihefovea centralis, the cones 

 have a diameter of 2-2. 5 jj. ; at the periphery 6-7 

 /* ; the rods have a diameter of about 2^. 



External to the layer of rods and cones there is a 

 layer of epithelial cells with delicate protoplasmic 

 processes which reach down between the rods and 

 cones. 



That the cones and rods are the retinal elements 

 I MATIC~REPRE- ^ or tne perception of light is proven by the shadows 

 SENTATION OF which the blood vessels of the retina cast. In a dark 

 THE RETINAL room a light is held to the side of the eye (at a, Fig. 

 NEURONS. ^^ while the other eye is c i ose d. The light illu- 

 minates the retina at the point b, which is found by drawing a line 

 from a through the nodal point K to the retina. From b light is 

 reflected by which other parts of the retina are illuminated ; the 



FIG. 33. 



cause of this sensation of light thus produced we refer to the exter- 

 nal world, hence we see the visual field dimly illuminated. One of 

 the rays reflected from b falls upon the retinal vessel g, and hence 



