STUDIES TN THE RETINA. 53 



their iutra-retinal terminal swellings.! Fig. 21 shows them 

 in the cod^ fig. 20, a, h, in the bleuny, fig. 22, a, h, c, in the 

 plaice, fig. 24, a, in the trout. These swellings, which are 

 also specially conspicuous in Ramon y Cajal's figures from 

 metal impregnation preparations, have hitherto found no 

 explanation. They can now be accounted for as the points 

 where the refractive matter escaping from the rods is 

 temporarily arrested as it reaches the outer reticular layer. 

 Confirmatory evidence can be seen in the fact that their size 

 depends upon the functional activity of the part. Fig. 22, 

 a, h, and c, shows three parts of the same retina. They show, 

 as do all the fish eyes I have examined, that the retina is 

 very unequally used up. The part a from the central region 

 shows the largest swelling of the inner limbs of the knobs of 

 the rod fibres, and the most marked using up of the inner 

 and middle nuclear layers. 



Having brought the refractive matter thus far in the eyes 

 of the fish, we may go back and consider some other eyes in 

 which it escapes from the outer limbs without swelling the 

 inner limbs to such monstrous proportions. We can only 

 refer to two cases, for a full treatment of the subject would 

 require a close comparative study of the retinas of all the 

 animal groups other than fish. The two cases chosen are 

 especially interesting because they present such striking 

 contrasts : (1) the Amphibia, with the inner limbs of their 

 adult rods quite small and insignificant as compared with the 

 outer limbs ; (2) the Primates, with their long, rather thick 

 inner limbs and thin outer limbs not much, if at all, longer 

 than the inner limbs (see fig. 31, a). 



(1) That the refractive matter escapes into the inner limbs 

 in the Amphibia we know from the invariable presence of 

 the ellipsoid. But the ellipsoid does not, as a rule, seem to 

 grow, so that if refractive matter is always exuding through 

 the transverse membrane, it must as rapidly be transformed 

 and conveyed away through the retina. Certain it is that 



^ I nctd oul}' refer to tiie familiar text-book diagrams, bueh as fig. 52, 

 p. iG, of ' Quaiu's Aiuitoiuy,' lOtli edition, vol. iii, part 3. 



