GENERAL ORGANOLOGY 



119 



stages the visual cells are closely related to accumulations of pigment 

 wliich occur either in or surrounding the cells. That this pigment is not 

 absolutely essential for light perception is shown by the visual powers of 

 albinos which are free from pigment, but it clearly must increase the 

 sensitivity of the cells, for pigmentation is so common that the simplest 

 eyes may be defined as sharply defined pigment spots, to which there is 

 frequently added a lens to concentrate the light (fig. 84, III). 



Eyes. — From such beginnings, which are evidently only intended to 

 recognize light and darkness, there are all transitions to the image-forming 

 eyes of the vertebrates and apparently the cephalopods. The retina is 

 rendered more efficient by the development of rhabdoms on the peripheral 

 ends of the visual cells, rod-like processes which aid in hght perception, 

 and in the vertebrates usually divided into rods and cones (fig. 85, 9). 



P 



Fig. 85. — Human retina (after Gegenbaur). P, pigment-Ia3-er; E, layer of 

 sensory cells; G, optic ganglion; i, limitans interna; 2, nerve-fibre layers; 3, ganglion 

 cells; 4, inner reticular layer; 5, inner granular layer; 6, outer reticular layer; 7, outer 

 granular layer; 8, limitans externa; 9, rods and cones; 10, tapetum nigrum; M, MuUer's 

 fibres. 



In the vertebrates and many invertebrates the retina contains a 

 reddish pigment, the 'visual purple,' which is quickly bleached in the 

 light and as quickly regenerated in darkness, and which apparently pla3-s 

 an important part in vision. In the course of the optic nerve there are 

 numerous ganglion cells which form an optic ganglion (figs. 85 and 356), 

 lying outside the eye in the invertebrates, in the vertebrates forming a 

 number of layers (G, fig. 85), inside the retina proper (E), which is 

 formed of the visual cells (outer granular layer) with the fibres of the 

 rods and cones and the rhabdoms themselves. 



Accessory Structures. — If a sharp image is to be cast on the retina, 

 the light rays coming from a point without the eye must be brought again 

 to a point on the retina by refractive substances (lens, cornea) ; therefore 

 there must be a space between the dioptric apparatus and the retina. The 



