SENSE OF SIGHT. 625 



and although these are still connected, by their fibrous extensions, with 

 the other parts of the retinal tissue. 



Finally the layer of rods and cones, at the situation of the macula lutea 

 and fovea, though preserving its general character, shows special features 

 by which it is readily distinguished from the corresponding parts else- 

 where. In this la^'er, over the greater portion of the retina, the rods 

 are the most abundant element, the cones being distributed among them 

 in smaller numbers. In the borders of the macula lutea (Fig. 195, B), 

 the cones become more numerous in proportion to the rods, and in the 

 fovea centralis (Fig. 195, (7) the layer is composed exclusively of cones. 

 At this part, the cones are longer than elsewhere, and more slender, so 

 that a larger number are comprised within an equal space ; and the 

 layer itself, consisting of elongated cones standing perpendicularly, is 

 increased in thickness, in proportion to the greater length of its con- 

 stituent elements. The thickness of the cones at their base, over the 

 retina generally, according to the measurements of Schultze, is a little 

 over 6 mmm., and their length less than 50 mmm. ; but at the fovea cen- 

 tralis their thickness is reduced to 3 or 3.5 mmm., while their length, 

 in the same situation, may reach 100 mmm. Each cone is connected, 

 here as elsewhere, through the nucleus and nucleus fibre of the pre- 

 ceding layer, with the other portions of the retina, and beyond doubt, 

 in some direct or indirect way, with the optic nerve fibres of its internal 

 layer. 



Thus the perception of light, in the act of vision, is a process con- 

 sisting of several successive acts. The luminous ray passes through 

 the transparent internal or superficial layers of the retina, until it 

 reaches the situation of the two outer layers. Here it produces a 

 change in the condition of the nervous elements, of whose nature we 

 are entirely ignorant. It might be compared with that which is caused 

 by the same agent in the sensitive film of a photographic camera ; but 

 this comparison would be only one of analogy, and would not imply 

 any identity of the ph3 r sical or chemical change produced in the two 

 cases. It would simply express the fact, which is undoubtedly estab- 

 lished, that the luminous ray, after traversing all the other transparent 

 and refracting media of the eye without leaving any trace of its passage, 

 on arriving at the two outer layers of the retina, excites in one or both 

 of them a kind of action which is the first step in the visual process. 

 This condition of the retinal elements then calls into activity the fibres 

 of the optic nerve, which in turn transmit the stimulus to their point of 

 origin in the brain. Thus far, there is no conscious perception, nor 

 even any nervous effect resembling in itself our idea of lurninositj^. 

 The retina itself is distinguished from other nervous tissues by being 

 sensitive to light ; that is, it may be thrown into a state of activity 

 under the influence of a luminous ray. But it has no other perception 

 of light than this, any more than the silvered film of a photographic 

 plate ; and, if the optic nerve be severed, blindness results, however per- 

 fect may be the condition of the retina. 



