RECAPITULATION 161 



The existence of photomechanical changes of the rods, 

 cones, and epithehal pigment in response to illumination 

 of the eye has long been recognized. These changes con- 

 sist of the forward migration of the epithelial pigment, con- 

 traction of the cones, elongation of the rods in the light, 

 and the inverse changes in darkness. Whereas these struc- 

 tural changes are extensive in the eyes of fishes, anuran 

 amphibians, and birds, they are slight in reptiles and their 

 occurrence has never been demonstrated with conviction 

 in the eyes of mammals and man. Although much has been 

 written concerning the supposed significance of these 

 changes in relation to adaptation of the eye to light and 

 darkness, the various explanations have never fully satisfied 

 all the conditions, due to the lack of uniformity of the 

 responses throughout the vertebrate groups. Chapter VI 

 gives a more or less detailed account of the photomechanical 

 responses including various factors which influence them 

 such as light, temperature, etc. Different theories held to 

 account for the adaptive significance of these responses are 

 taken up in Chapter VII where it is pointed out that none 

 offer an all-inclusive solution. 



Although there are a number of factors which influence 

 visual acuity, it is apparent that one of these, and an impor- 

 tant one, pertains to the size and numbers of visual cells 

 per unit area in the retina. A low visual acuity signifies that 

 the average distance between the active retinal elements is 

 large, whereas a high visual acuity indicates that the distance 

 is small. It is obvious, therefore, that vertebrate retinas 

 must vary greatly in this respect. Since the number of rods 

 and cones is fixed structurally in any given area of the retina, 

 it is apparent that they must possess different physiological 

 thresholds in order to mediate the great variations in visual 

 acuity accompanying changes in illumination. 



In most vertebrate animals some degree of retinal special- 

 ization occurs in the fundus for increased visual acuity. This 

 may be very slight in some forms and involve merely a 

 greater crowding of the elements per unit area in this region. 



