356 NORMAL HISTOLOGY. 



conclusion inferable from the distribution of these elements in the hu- 

 man retina, that the cones are the essential perceptive instruments, is 

 not applicable as a generalization, since in many of the lower animals 

 the cones are in the minority or even entirely wanting (hedgehog, 

 shark, sturgeon), and the rods predominate ; it seems, however, 

 probable that the highest acuity of vision requires the presence of 

 cones. The entire number of cones in the human retina has been 

 computed at something over three and one-half millions (Salzer), while 

 the rods are supposed to aggregate one hundred and thirty millions. 



The pigment-layer represents the outer lamina of the embry- 

 onal optic vesicle, and consists of a single layer of polyhedral epi- 

 thelial cells containing pigment-granules in varying amount. These 

 cells ( 1 2-1 8 ft) are usually six-sided, but may have fewer or more 

 borders ; the cells in the vicinity of the ora serrata are of exception- 

 ally large size and dark color. The elements of tne pigment-layer 

 exhibit a differentiation into an outer zone next the choroid, free 

 from pigment and containing an oval nucleus, and an inner zone 

 loaded with pigment-granules. 



The inner part of the pigment-cells includes protoplasmic pro- 

 cesses directed towards the layer of neuro-epithelium, between 

 the rods and the cones of which they extend for a variable distance ; 

 the depth to which the pigment-granules penetrate along the pro- 

 cesses between the cells depends upon the influence of light, since 

 under strong illumination the granules wander along the protoplas- 

 mic processes as far as the inner segment of the rods and the cones, 

 while in eyes kept in the dark for some time before death the intercel- 

 lular processes remain uninvaded. 



The structural details above described represent the construction 

 of the retina throughout the greater part of its extent : two regions, 

 however, present such marked variations from the typical arrange- 

 ment as to call for brief special mention ; these are the macula lutea 

 and the ora serrata. 



The macula lutea and the contained fovea centralis corre- 

 spond to the posterior pole of the visual axis, and are distin- 

 guished physiologically by the acuity of vision, which here attains 

 its highest degree. The macula lutea is characterized, in addition 

 to its yellow color, by a distinct thickening of certain of the retinal 

 layers and by the absence of the rod-visual cells within its area. 

 The distinctive color of the macula depends upon the presence 

 of diffuse yellowish pigment within the layers internal to the 

 visual cells, the latter elements remaining colorless ; in consequence 

 of this arrangement the fovea, in which the neuro-epithelium alone 

 exists, is devoid of pigment, and therefore appears as a light spot 

 within the colored area. 



