THE FIELD OF VISION 



31 



respectively forward and backward. A large oval pupil, while 

 affording a wide range of lateral vision, must detract very 

 considerably from the clearness of vision by not having the 

 stenopaic effect of a round pupil. I would suggest that one 

 function of the corpus nigrum is at times to compensate for 

 this defect by the conversion of the oval pupil into two 

 round ones. 



Animals, such as the terrestrial herbivorous mammals, 

 who require panoramic vision, have their eyes set laterally 

 in the head so as to obtain the largest circumferential effect 

 of the combined monocular fields. Lindsay Johnson 3 

 measured the divergence of the optical axes in a large number 

 of representative animals, and constructed a diagram 

 graphically showing how it varies in the different natural 

 orders, families, genera and species. From this it is seen 

 that the most laterally placed eyes are met with among the 

 Kodentia, the Marsupialia and the Ungulata. The greatest 

 divergence is in hares, whose optical axis in each eye measures 

 85° of divergence from the middle line. It seems probable 

 that they, and some of the other rodents, have complete 

 panoramic vision, i. e., are able to see in their circumference 

 at one and the same time. 



Among the Carnivora the smallest amount of divergence 

 is in lions and cats, in which it is less than 10° in each eye. 

 The Simise and man alone among mammals have parallel 

 optic axes. 



This movement forward of the optical axes toward 

 parallelism is clearly in the interests of binocular vision, and 

 at the sacrifice of the range of simultaneous circumferential 

 vision.* As ontogeny is a condensed recapitulation of 

 phylogeny, it is interesting to note that in the human embryo 

 the optic vesicles when first formed are directly opposite 

 to one another, from which position they gradually turn 



