CHAP, in.] SIGHT. 63 



exception of the nuclei of the fibres of M tiller, which, as we have 

 said, are placed in this layer, belong to cells in which the amount 

 of cell-substance is in most cases small compared to the nucleus. 

 These cells are arranged in several tiers and, though possessing a 

 general resemblance to each other, are not all of the same kind. 



In the cells (Fig. 145, 1). p.) which form the greater number of 

 the tiers, all in fact except the innermost and outermost tiers, the 

 nucleus, round and highly refractive, is surrounded by a thin 

 layer of cell substance, which is prolonged in a radial direction- 

 from the opposite poles of the nucleus into an outer or peripheral 

 and an inner or central process. The peripheral process, directed 

 straight towards the outer molecular layer, soon branches and 

 dividing into fine fibrils is lost to view in that layer; in some 

 animals, however, the process is said to give off a branch which, 

 continued undivided through the outer molecular layer and the 

 outer nuclear layer ends abruptly just inside the external limiting 

 membrane in a club-shaped swelling. The central process, thinner 

 and more delicate than the peripheral one, and frequently varicose, 

 is directed straight inwards to the inner molecular layer, and, after 

 traversing that layer for some distance without dividing, is lost 

 to view, or, according to some observers, ends in a peculiar tangle 

 of fine nervous fibrils in the inner zone of the layer (cf. Fig. 145). 

 From their possessing two obvious processes running in two 

 opposite directions these cells have been called the bipolar cells 

 of this layer. 



The cells of the innermost tier (Fig. 145, u. p.), whose nuclei 

 are also round and refractive with conspicuous nucleoli, vary more 

 in size than do those of the outer rows, and are on the whole 

 larger. The cell-substance around each nucleus is continued 

 not into two processes, but into a single one only, which is 

 directed inwards into the inner molecular layer and is there lost 

 to view, or, according to some observers, divides into fine fibrils in 

 the outer zone of the layer (Fig. 145) ; the cells have therefore 

 been called unipolar to distinguish them from the bipolar cells 

 just described. They have also been called " spongioblasts " and 

 have been supposed to be concerned in the maintenance of the 

 neuroglial framework ; but they are probably nervous in nature. 

 Some observers have described them as giving off besides the 

 branched process just described an undivided axis cylinder process 

 (Fig. 145 y), which, running through the whole thickness of the 

 inner molecular layer, joins the layer of optic fibres and indeed 

 becomes an optic fibre. 



A third kind of cell has been described as forming or contri- 

 buting to the outermost tier of nuclei of this inner nuclear layer 

 just inside the outer molecular layer. The body of a cell of- 

 this kind (Fig. 145, b. c.), somewhat flattened in the plane of 

 the retina, is said to give off on the outer side processes which, 

 running transversely and soon branching, are lost to view as fine 



