562 



PHYSIOLOGY 



(#) The point of entry of the optic nerve, \vhere the whole thickness 

 of the retina is composed of nerve fibres, is absolutely insensitive to light 

 and constitutes the blind-spot (Fig. 288). If the light of a small flame be 



FIG. 288. 



directed, by means of a mirror, into the eye so that it falls only on to the optic 

 disc, the individual receives no sensation of light. The existence of the 

 blind- spot is more easily shown by the following experiment : On closing the 



left eye and gazing fixedly with the right .eye 

 at the white cross in the figure, on appi v . 

 mating the book to the eye a point will be 

 found, when the book is a/Ta distance T^feTght 

 inches from the eye, at which the white circle 

 becomes invisible and the whole figure appears 

 to be covered by the black ground. By 

 measuring the apparent size of the blind-spot 

 and its distance from the point of fixa^on; 

 we find that its situation on the retina corre- 

 sponds exactly to the point of entry of the 

 optic nerve. The blind-spot is so large that 

 at a distance of about six feet the image of 

 the head of a man will fall on it and therefore 

 be invisible. 



(b) At the point of most distinct vision, 

 i.e. the fovea centralis, all the layers of the 

 retina are absent except the outermost, i.e. 

 that of the cones. 



(c) ' Purkinje's figures.' If a strong light 

 be focused by means of a lens on to the sclerotic 

 just outside the cornea, and the eye be made to 



stare fixedly at a dull background, an arborescent image of the retinal, vessels 

 will appear on the background. On moving the illumination the image of the 

 vessels will move in the same direction. Knowing the dimensions of the 

 eyeball and the distance of the background from the eye as well as the angle 

 through which the light is moved and the apparent displacement of the image 

 of the vessels, the distance of the sensory part of the retina behind the vessels 

 may be calculated (Fig. 289). Direct measurements in this way have shown 



FIG. 289. Diagram of the path 

 of the rays of light in the for- 

 mation of Purkinje's figures. 

 v represents a retinal vessel. 

 When this is lluminated from A, 

 a shadow is formed on the hinder 

 layers of the retina at a'. This 

 is projected along a line passing 

 through the optic axis, and ap- 

 pears to come from a point (a") 

 on the wall. On moving the 

 light from A to B, the image of 

 the vessel appears to move from 

 a" to &". 



