376 THE BODY AT WORK 



as the retinal image is reversed, a movement from right to 

 left is interpreted by consciousness as a movement from left 

 to right. Given the angle through which the light is moved, 

 an<I the apparent displacement of the shadows, it is a simple 

 matter to calculate the distance behind the bloodvessels of the 

 sensitive layer of the eye. So definite are Purkinje's figures 

 that the shadows of individual blood-corpuscles can be fol- 

 lowed, and the rate at which they are moving in the capillaries 

 of the retina calculated. 



The retina is the organ of vision. Cornea, iris, lens, vitreous 

 humour, are parts of the camera in which this sensitive screen 

 is exposed ; and of the retina, the sensitive layer is the layer 

 of rods and cones. Interest therefore centres in these struc- 

 tures. They are disposed with the utmost regularity on the 

 posterior surface of a thin, reticulated membrane the outer 

 limiting membrane. But rods and cones are only the outer 

 halves of sensory cells, the inner portions of which, reduced to 

 a minimum in thickness, except where they contain their 

 nuclei, lie in the outer nuclear layer. Eods are the larger 

 elements. Each consists of an outer segment, or limb, of 

 relatively firm substance transversely striated, and liable to 

 break into discs ; and an inner limb of much softer substance, 

 again divisible into two parts, the outer longitudinally striated, 

 the inner granular. Cones are almost identical in structure 

 with rods, save that their outer limbs are much smaller, their 

 inner limbs rather fuller. In frogs and various other animals, 

 but not in Man, each cone contains at the junction of its two 

 limbs a highly refracting globule of oil, often brightly coloured, 

 red, yellow, or green. 



The layers in front of the rods and cones contain nervous 

 elements accessory to them. In the " inner nuclear layer " 

 are the ganglion-cells of the retina, homologous with the cells 

 of the ganglia on the posterior roots of spinal nerves ; but, in 

 the retina, bipolar and extremely minute. On either side of 

 the rather thick layer occupied by the nuclei of these ganglion- 

 cells (and of cells of other types which, for the sake of clearness, 

 we omit) is a felt-work of nerve-filaments in which their two 

 extremities arborize. The most internal, or anterior, layer 

 consists of a single sheet of rather large collecting cells and of 

 their axons, which stream towards the optic nerve. Each 



