152 VETERINARY PHYSIOLOGY 



the left eye, fixing the right eye on the left-hand mark with 

 the paper held at such a distance from the eye that both 

 marks are visible, then bringing the paper nearer to the eye, 

 when the right-hand mark will first disappear. When the 

 paper is brought still nearer it will reappear (fig. 72, a) 

 (Practical Physiology). 



2nd. By making a mark on a sheet of paper, and, with 

 the head about a foot from the paper, moving the point of a 

 pencil to the right for the right eye, or to the left for the left 

 eye, when the point will disappear and again reappear. 

 The eye is blind for all objects in the shaded region 

 (lig. 72, a, h) {Practical Physiology). By resolving the 

 various triangles, the distance of the blind spot from the 

 central spot of the eye may be determined, and the 

 diameter of the blind spot may also be ascertained. 

 Hence subtending this blind spot is a region in which objects 

 are not seen (fig. 72, a, h). 



The shape of the blind spot may be mapped out by 

 fixing the head about a foot from the paper, moving the 

 point of the pencil out till it disappears, and then moving it 

 in different directions and marking when it reappears (fig. 72, h). 

 It is never quite circular, and often shows rays extending 

 from its edge which are due to the blood-vessels (Practical 

 Physiology). 



(2) The Field of Vision.— The rest of the retina, forward to 

 the ora serrata, is capable of stimulation. 



(3) The layer of the retina capable of stimulation is the 

 layer of rods and cones. This is proved by the experiment 

 of Purkinje's images, which depends upon the fact that, if a 

 ray of light is thrown through the sclerotic coat of the eye, 

 the shadow of the blood-vessels stimulates a subjacent layer 

 (fig. 73, c), and the vessels appear as a series of wriggling 

 lines on the surface looked at. If the light be moved, the 

 lines seem to move, and, by resolving the triangles, it is 

 possible to calculate the distance behind the vessels of the 

 part stimulated, and this distance is found to correspond to 

 the thickness of the retina. The shadows of the blood- 

 vessels are not seen in ordinary vision, because they then fall 



