458 H. M. BERNARD. 



the retina, or that it is manufactured in situ within the 

 rods. 



That it does not come from the retina, from which we can 

 easily trace the fluid and the staining netwoi'k, we gather 

 from the total absence of any refractive matter in the inner 

 limb except in the ellipsoid; and, as we shall presently see, 

 the position of this body forms additional evidence that the 

 source of the refractive matter is from without inwards to- 

 wards the retina, and not from the retina outwards. 



That the matter is not manufactured in situ we gather 

 from the microscopic appearances, which show very clearly 

 that it is forced in through the walls. The evidence for this 

 is to be seen iu the changes already described, which take 

 place in the character of the reticulum within the outer limb 

 of the rod. Figs. 4, h, and 6, h, show this reticulum simply 

 diffused equally across the section; figs. 13, c — h, and 12 

 show different stages in its compression towards the axis 

 of the rod. Now it is difficult to explain this compression 

 except on the assumption of some matter passing in through 

 the walls and crushing it inwards, stretching, or perhaps 

 merely lengthening the threads which attach it to the walls. 

 Fig. 13,7, shows the process as being irregular, while fig. 14 

 shows that it may take place locally, i. e. along one side of 

 a rod and not on the opposite side. This observation is im- 

 portant, because it is in keeping with the fact that the 

 tongues of the pigment cells run up lengthwise between the 

 rods. Fig. 13, i, shows that at times the reticulum, though 

 compressed towards the axis, may retain some of its concentric 

 threads, the refractive matter passing them by. The refrac- 

 tive layer was here I'b fj. thick, the whole rod being 9 /m.^ 



Again, in eyes in which, after exposure to light, the pig- 

 ment has been forced up to the membrana limitans externa, 

 individual granules can be seen remaining behind after the 

 general retreat of the pigment, and sticking to the clear 

 protoplasmic walls of the inner limbs. Many of them can 



1 Zenker ('Arch. mikr. Anat.,' iii, 1867, p. 259) discovered tbat the outer 

 layer of the rod is more highly refractive than the axial portion. 



