540 



COMPARATIVE PHYSIOLOGY. 



effected in the retina as sensory impressions or impulses, which, 

 when completed by corresponding changes in the brain, develop 

 into sensations, which are represented psychically hj percep- 

 tions ; hence, though all these have a natural connection, they 

 may for the moment be considered separately. It is as yet 

 beyond our power to explain how they are related to each 

 other except in the most general way, and the manner in which 

 a mental perception grows out of a physical alteration in the 

 molecules of the brain is at present entirely beyond human 

 comprehension. 



Affections of the Retina.— There is no doubt that the fibers of 

 the optic nerves can not of themselves be directly affected by 

 light. This may be experimentally demonstrated to one's self 

 by a variety of methods, of which the following is readily car- 

 ried out: Look at the circle (Fig. 389) on the left hand with the 



Fig. 389 (after Bemetein). 



right eye, the left being closed, and, with the page about twelve 

 to fifteen inches distant, gradually approximate it to the eye, 

 when suddenly the cross will disappear, its image at that dis- 



FiG. 390.— ■Diagrammatic section of macula intea in man (after Huxley), a, a,'*pigmeBt 

 of choroid; 6, c, ft, c, rods and cones; d, a, outer granular or nuclear layer; f,f, 

 inner granular layer; g, g, molecular layer; A, A, layer of nerve-cells; i, i, fibers or 

 optic nerve. 



