348 PHYSIOLOGY CHAP. 



kept in darkness, are so small (G'l-0'2 m.) that they may be 

 reckoned as errors due to fixation. 



Equally doubtful are the observations on the displacement of 

 pigment in mammals generally. Angelucci (1878) found that in 

 rabbits including albinos exposed to light it was difficult to 

 dissect away the epithelial layer from the underlying layer of the 

 rods after the eye had been fixed in osmic acid, without tearing the 

 outer portion of the rods ; this did not occur when the eye had 

 been kept in the dark. In dogs, also, Chiarini observed this 

 greater cohesion between the retinal epithelium and the rods 

 and cones in animals exposed to sunlight. This fact does not 

 depend on the wandering of pigment, because it also occurs when 

 the epithelium contains more. So that structural changes due 

 to light are not merely inconspicuous in the retinal elements 



FIG. 171. Vertical section of temporal half of retina of Corvus comix, fixed in v. Tellyesniesxky's 

 fluid, magnified 510 diameters. (P. Chiarini.) A, after keeping the animal in the dark for 

 24 hours ; B, after exposing it to direct sunlight for 6 hours. 



of mammals, as Angelucci stated, but are, as Chiarini says, 

 rudimentary. 



The fact that in the higher animals, in which vision is most 

 developed and differentiated, the movement of the fuscin and con- 

 traction of the cones either do not occur, or are rudimentary, 

 prevents the inclusion of these phenomena as essential factors in 

 a general theory of vision. But this does not preclude us from 

 studying the nature and functional value of the objective changes 

 with which vision is associated in the lower animals. 



The wandering of fuscin from the epithelial cells into the 

 protoplasmic processes between the rods and cones upon exposure 

 to light cannot be considered as a passive displacement due to the 

 COLI traction of these cells as assumed by Angelucci. If this 

 were true, the body of the epithelial cell could never remain 

 completely destitute of pigment, as is the case after prolonged 

 exposure to direct sunlight. Nor is it more accurate to regard it 



