238 ANIMAL PHYSIOLOGY. 



the addition of lens and camera, which are probably necessi- 

 tated by the enormous increase in number of the sensitive 

 points; for every rod or cone of the bacillary layer is a 

 separate nerve-termination. In the eye of the cuttlefish, the 

 bacillary layer is the part of the retina, turned towards the 

 lens, and is, probably, like the lens, a development of the 

 cuticle; but in the vertebrate eye, we have seen that it is 

 the part of the retina lying against the choroid, and is a 

 development of the brain. 



175. If the analogy of nerve-terminations in other organs 

 be attended to, it will be at once perceived that those of the 

 retina are the rods and cones of the bacillary layer ; and a 

 variety of other considerations show that they really are so. 

 One might naturally expect that the surface of the retina 

 which is turned towards the light, would be the one to be 

 affected by the rays impinging on it ; but this is not the case. 

 For the part next the light is the layer of ramifying fibres of 

 the optic nerve, and the spot where those fibres are most 

 numerous is the optic pore, which happens to be wholly 

 insensible to light. 



Ihis insensibility of the optic pore can be easily proved. 

 The axis of the eye is always directed to the object looked 

 at; in that axis lies the yellow spot, and to its inner side 

 is the optic pore, receiving the rays entering from a point 

 external to the object looked at. If, now, the left eye 

 be shut, and the right eye fixed on the cross here 

 represented, while the book is slowly moved towards and 

 away from the eye, at a certain distance the round mark to 



the right will suddenly disappear, to come again into view, 

 as the book is brought nearer or held further off. The same 

 result is obtained, if the experiment be made with white 

 spots on a dark ground, or with colours; and the explana- 

 tion obviously is that, at a certain distance, the round mark 

 falls on a spot, internal to the axis, which is insensible to the 

 presence or absence of light. 



This experiment also shows the importance of the yellow 

 spot as the seat of clear vision, that spot from which the 

 fibres of the optic nerve are absent, and in which the rods of 



