THE SENSES 539 



cones. Here they give rise to a nervous impulse which retraces 

 its steps in the retina, until it arrives at the layers next the 

 vitreous humour, from which it is carried off by the optic nerve 

 to the brain. It is in the rods and cones that the primary con- 

 version of light-vibrations into visual impulses is effected. Each 

 cone is connected with a single nerve cell, but there may be 

 several rods to one nerve cell ; the cone is, therefore, considered 

 to offer a more direct conducting path than the rod. In the 

 human eye cones exist over the area of acute vision. Cones are 

 especially adapted for daylight vision (see p. 561) ; further, it is 

 believed they are the seat of colour perception. 



Visual Purple, or Rhodopsin, is a curious red pigment existing 

 in the retina ; it is found in the rods, but not in the cones. This 

 colouring matter is readily decomposed by light, and is conse- 

 quently always being produced. Its function in connection with 

 the theory of vision is considered at p. 561. 



Optic Disc. — The entrance of the optic nerve within the eyeball 

 is spoken of as the optic disc or papilla ; it is a concave oval surface, 

 measuring in the horse 3-5 mm. deep and 5 mm. wide, surrounded 

 by a white ring formed of sclerotic (see Plate I., Fig. 4). It lies* 

 in the horse, towards the bottom of the eyeball and inclined to 

 the temporal side. This region is blind, owing to the absence 

 of both rods and cones. On the temporal side of the optic disc 

 in man and the higher apes is a yellow area, the macula lutea 

 or yellow spot, which is about 6 mm. in diameter. It is now 

 believed that the yellow tint of this area is a post-mortem change, 

 for it cannot be seen in the living eye. Within the yellow spot 

 is a small depression, known as the fovea centralis, having a 

 diameter of 0-3 to 0*4 mm. At this point the retina is thinned 

 out until nothing but the cones are left. This is the area of 

 acute vision, and it is situated in the centre of the field of vision. 

 Visual acuity diminishes rapidly as the image falls away from 

 the fovea. There is no yellow spot or fovea centralis in the 

 lower animals, but there is, no doubt, a region where vision is 

 most acute. Most animals raise the head when staring intently 

 at an object. This is especially well seen in the horse, who, by 

 raising the head very high and protruding the muzzle, renders 

 the face more horizontal, and doubtless brings the object on to 

 the most receptive part of the retina. In man a line drawn from 

 the yellow spot to the centre of the cornea is called the visual 

 axis of the eye. The visual axis does not agree with the optic 

 axis — viz., a line drawn exactly through the centre of curvature of 

 each refractive medium. 



The Optic Nerve arises from the retina, and the first portion 

 of its path ends by forming synapses with the cells in the external 

 geniculate body, the posterior portion of the optic thalamus, 



