No. 3-] ACUTE VISION IN VERTEBRATES. 483 



probably visible to the naked eye. In horizontal sections the 

 area and fovea may be readily seen. The fovea is broad and 

 shallow when compared with that of most birds and some 

 reptiles. It is located on the temporal side, about midway 

 between the nerve entrance and the ora serrata, and a little 

 above. A horizontal section through the nerve passes through 

 the area below the fovea, as shown in PI. XXX, Fig. 59 (i). 

 A section through the fovea is shown in Fig. 58 (i). 



Physiological. 



In order to make a physiological comparison of the areas of 

 acute vision in the different vertebrates, the exact function of 

 the different elements of the retina must be known. Most 

 physiologists agree on the functions of all the elements except- 

 ing the rods and cones. All are agreed, however, that the rods 

 and cones are the elements which give the sensation of sight, 

 but just the function of each is very obscure. The source of 

 information regarding the functions of the rods and cones has 

 necessarily been confined to man. When this has been finally 

 settled, a more accurate comparison of the powers of sight in 

 the different vertebrates can be made. 



A great many theories have been advanced regarding the 

 functions of the rods and cones, and as these theories cannot 

 be fully verified or tested by physiological experiments, they 

 will have to be accepted as such. 



What the changes are which take place in the retina during 

 an act of sight had long been a mystery till the visual purple 

 was discovered in the rod and cone layer by Boll. This, how- 

 ever, sufficed for only a short time, as it was soon found that 

 the cones possessed no visual purple, or at least none could be 

 demonstrated in them. Since the cones are the only sensitive 

 elements in the fovea, some other photo-chemical substance 

 must be present in them. The theories of Young-Helmholtz, 

 Herring, Mrs. Franklin, etc., agree generally in the functions 

 of the rods and cones, but differ in the photo-chemical sub- 

 stance and its change in an act of sight. Since the theories of 

 Youngc-Helmholtz and Herring do not ascribe different func- 



