CHAP. IIL] , SIGHT. 1199 



inner molecular, and V. the outer molecular layer, n. the nucleus of the fibre 

 of Miiller. 



On the left hand side the nervous elements are represented. A rod with its r.o. 

 outer, and r.i. inner limb, continued on as the rod fibre r.f. with its banded 

 nucleus r.n. ; also a cone with its outer c.o. and inner c.i. limb, and cone 

 fibre c.f. with the nucleus c.n. 



In IV., the inner nuclear layer, are represented, (1) one of the bi-polar cells, b.p., 

 with its outer process lost in the outer molecular layer, and its inner process 

 ending in a tangle in the inner zone of the inner molecular layer ; (2) one of 

 the so-called unipolar cells u.p., with its single process branching into the outer 

 zone of the inner molecular layer, the possible axis cylinder process of such a 

 cell being also shewn at y ; (3) one of the cells b.c. in the outermost tier of the 

 nuclear layer, branching on the outer side into the outer molecular layer, and 

 sending inwards an 'axis-cylinder process' x to join the layer of optic fibres. 

 It will be remembered that by far the greater number of the nuclei of the inner 

 nuclear layer belong to the first of these three kinds of cells. 



In II. is shewn a ' ganglionic ' cell g.c. with its axis cylinder process becoming 

 an optic fibre, and the branches of the cell body lost to view as fine processes 

 in the middle zone of the inner molecular layer. Optic fibres are also shewn 

 at op. f. 



The inner end of the inner limb, piercing the external limiting 

 membrane, narrows rapidly to a delicate thread r.f., which is 

 directed straight inwards towards the outer molecular layer, is 

 often varicose, and has otherwise much the appearance of a fine 

 nervous filament. This process of the inner limb, known as the 

 rod-fibre, bears at some part of its course, sometimes nearer 

 the external limiting membrane, sometimes nearer the outer mole- 

 cular layer, an oval nucleus r.n. over which the fibre expands to 

 form a very thin layer of cell-substance. The nucleus is peculiar 

 inasmuch as its staining, chromatin, constituents are, at least 

 frequently, arranged in transverse bands, giving the stained 

 nucleus a banded appearance, like that of the planet Jupiter. 



At the outer molecular layer the rod-fibre is lost to view ; 

 whether, as some think, it ends abruptly at the layer in a knob- 

 like termination, or changing its direction and running transversely 

 ends in fine fibrils, thus contributing to the network of the layer, 

 or again whether, as others think, it crosses the layer and passes 

 into the inner nuclear layer cannot at present be regarded as 

 settled. 



Obviously we may consider the 'rod,' taken as a whole, to be 

 an elaborated epithelium cell, whose nucleus lies on its inner 

 thread-like continuation or rod-fibre on the' inner side of the 

 external limiting membrane and contributes to the nuclei of 

 the outer nuclear layer, and whose main body, lying outside the 

 external limiting membrane, is sharply divided into an inner 

 part, inner limb, which remains more or less protoplasmic in 

 nature, and into an outer part, outer limb, which has undergone 

 a differentiation having a certain resemblance to cuticular dif- 

 ferentiation but still of a distinctly peculiar kind. Between the 

 outer and inner limb there appears to be a distinct break; the 

 two are in apposition but are not continuous; and indeed in 



