90 SPECIAL SENSES. 



fraction by the crystalline lens, however, is more complex in 

 its mechanism ; depending, first, upon the curvatures of its 

 two surfaces, and, again, upon the differences in the consist- 

 ence of different portions of its substance. In view of these 

 facts, we may simplify the conditions of refraction in the eye 

 by assuming the following arrangement : 



The cornea presents a convex surface upon which the rays 

 of light are received. At a certain distance behind its an- 

 terior border, is the crystalline, a double-convex lens, corrected, 

 sufficiently for all practical purposes, both for spherical and 

 chromatic aberration. This lens is practically suspended in a 

 liquid with an index of refraction equal to that of pure 

 w r ater ; as both the aqueous humor in front and the vitreous 

 humor behind have the same refractive power. Behind the 

 lens, in its axis and exactly in the plane at which the rays of 

 light are brought to a focus by the action of the cornea and 

 the lens, is the fovea centralis, which is the centre of distinct 

 vision. The anatomical elements of the fovea are capable of 

 receiving visual impressions, which are conveyed to the brain 

 by the optic nerves. All impressions made upon other por- 

 tions of the retina are comparatively indistinct ; and the point 

 of entrance of the optic nerve is insensible to light. 1 Inas- 

 much as the punctum caecum is situated in either eye on the 

 nasal side of the retina, in normal vision rays from the same 

 object cannot fall upon both points at the same time. Thus, 

 in binocular vision, the insensibility of the punctum caecum 

 does not interfere with sight ; and the movements of the 

 globe prevent any notable interference in vision, even with 

 one eye. The sclerotic coat is for the protection of its contents 



1 Some writers state that the punctum caecum possesses a faint sensibility to 

 light, referring to the old experiments of Brewster made by throwing the image 

 of a candle-flame upon the blind spot, and to recent observations with the oph- 

 thalmoscope, in support of this assertion. (LONGET, Traite de physiologic, Paris, 

 1869, tome ii., p. 904.) These observations, however, are fallacious, for the rea- 

 son that it is impossible to exclude impressions made by the diffusion of light to 

 the sensitive portions of the retina. The weight of experimental evidence is 

 decidedly in favor of the absolute insensibility of the blind spot. 



