No. 3-] ACUTE VISION IN VERTEBRATES. 447 



the eyes of some reptiles and demonstrated the presence of a 

 well-defined fovea in the lizard (Lacerta scutata) and the cha- 

 meleon. Joh. Miiller (12) says: " A foramen centrale is present 

 in the middle point of the retina of other reptiles, which is not 

 visible as in man, where the limiting membrane is unharmed, 

 but where the choroid shows through." H. Miiller (13), who 

 so admirably describes the eye of the chameleon and the retina 

 of the different classes of vertebrates, describes a well-defined 

 fovea found in many birds, while in some birds two foveas are 

 present. He also states that in mammals an area centralis is 

 present, which approaches in structure the yellow spot of the 

 human retina. 



Three things were always sought for by the early investiga- 

 tors : the yellow spot, the foramen, and the folds of the retina, 

 which more or less concealed the foramen. Though Home had 

 described these folds as due to post mortem changes and not 

 present in the fresh eye, they were considered as normal by 

 many writers even as late as the middle of the present cen- 

 tury (i). 



The old theory that the fovea was a foramen which enlarged 

 and contracted with the intensity of the light, thus protecting 

 the retina from injury, rapidly gave place with the use of the 

 microscope to the opposite view, that it was the place of acute 

 vision. The microscope further brought out the fact that the 

 cells of the yellow spot had a definite arrangement, and that 

 this arrangement might be present without a fovea. With this 

 thought in view investigations were made in all classes of ver- 

 tebrates with the result that a fovea has been found in each 

 class, and that an area centralis is quite common. 



Hulke (14) has described the presence of a point in the retina 

 of several amphibians and reptiles which, owing to a similar 

 arrangement of cells, he thinks corresponds to the human fovea. 

 Gulliver (15) has described the presence of a fovea in the fish, 

 and Carriere (16) in Hippocampus. Hoffmann (17) describes 

 an arrangement in the crocodile which corresponds to that in 

 the fovea. Krause (18) treats of the eyes of different verte- 

 brates, and states that the dove and cat possess a fovea, while 

 the chicken and dog do not. He seems to be the only person 



