128 



VETERINARY PHYSIOLOGY 



(3) Astigmatism is a defect due to unequal curvature of 

 one or more of the refracting surfaces in different planes. If 

 the vertical curvature of the cornea is greater than the hori- 

 zontal, when a vertical line is looked at, horizontal lines will 

 not be sharply focussed at the same time (fig. 59). 



B. Stimulation of the Retina 



1. Reaction to Varying Illuminations. (1) The Blind Spot. 

 At the entrance of the optic nerve the retina cannot be 

 stimulated because there are no end organs in that situation. 



FIG. 60. Methods of demonstrating the Blind Spot : a, by Mariotte's 

 experiment ; b, by moving a pencil along a sheet of paper. 



The existence of such a blind spot may be demonstrated 

 1st, By Mariotte's experiment; which consists in making two 

 marks in a horizontal line on a piece of paper, closing the 

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

 the paper held at a distance from the eye, when both mark& 

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

 the right-hand mark will first disappear, and, when the paper 

 is brought still nearer, will reappear (fig. 60, a) (Practical 

 Physiology) ; 2nd, By making a mark on a sheet of paper, and 

 with the head close to the paper moving the point of a pencil 



