THE KYK. 901 



are named rod-granules and cone-granules. The rod-granules are much the more 

 numerous, and are placed at different levels throughout the layer. They present a 

 peculiar cross-striped appearance, and have prolonged from either extremity a fine 

 process : the outermost is continuous with a single rod of Jacob's membrane; the 

 innermost passes inward toward the outer molecular layer, and terminates in an 

 enlarged extremity, from which are given off a number of minute fibrils, which 

 enter the outer molecular layer. In its course it presents numerous varicosities. 

 The cone-granules, fewer in number than the rod-granules, are placed close to the 

 membrana limitans externa. and are closely connected with the cones of Jacob's 

 membrane. They do not present any cross-striping, but contain a pyriform 

 nucleus, which almost completely fills the cell. From their inner extremity a 

 thick process passes inward to the outer molecular layer, where, like the processes 

 of the rod-cells, it terminates in an enlargement, from which are given off numerous 

 fine fibrils which enter the outer molecular layer. 



8. The Membrana Limitans Externa. This layer, like the membrana limitans 

 interna. is derived from the fibres of Miiller, with which structures it will be 

 described. 



9. Ji.n.-nb' $ Membrane (bacillary layer). The elements which compose this 

 layer are of two kinds, rods and cones, the former being much more numerous than 

 the latter. The rods are solid, of nearly uniform size, and arranged perpendicularly 

 to the surface. Each rod consists of two portions, an outer and inner, which are 

 joined together by a cement-substance and are of about equal length. They differ 

 from each other as regards refraction and in their behavior with coloring 

 reagents, the inner portion becoming stained by carmine, iodine, etc., the outer 

 portion remaining unstained. The outer portion of each rod is marked by transverse 

 stride, and is made up of a number of thin disks superimposed on one another. It 

 also exhibits faint longitudinal markings. The inner portion of each rod at its 

 inner extremity, where it is joined to the processes of the rod-granules, is indistinctly 

 granular : at its outer extremity it presents a fine longitudinal striation. being 

 composed of fine, bright, highly refracting fibrils. 



The cones are conical or flask-shaped, their broad ends resting upon the 

 membrana limitans externa. the narrow pointed extremity being turned to the 

 choroid. Like the rods, they are made up of two portions, outer and inner ; the 

 outer portion being a short conical process, which, like the outer segment of the rods, 

 presents transverse stria?. The inner portion resembles the inner portion of the 

 rods in structure, presenting an outer striated and an inner granular appearance, 

 but differs from it in size, being bulged out laterally and presenting a flask shape. 



10. The Pigmentary Z<///- /. / Tapettun ^ic/nun. The most external layer of 

 the retina, formerly regarded as a part of the choroid, consists of a single layer of 

 hexagonal epithelium cells loaded with pigment-granules (Fig. 21). In the eyes of 

 albinos the cells of the pigmentary layer are present, but they contain no coloring 

 matter. In many of the mammals also, as in the horse, and many of the carnivora, 

 there is no pigment in the cells of this layer, and the choroid possesses a beautiful 

 iridescent lustre, which is termed the tapetum lucidum. 



nectit'e-tissue Framework of the Retina. Almost all these layers of the 

 retina are connected together by a sort of supporting connective tissue, which has 

 been named the fibres of J/// //</. or radiating fibres, from which the membrana 

 limitans interna et externa are derived. These fibres are found stretched between 

 the two limiting layers. " as columns between a floor and a ceiling." and passing 

 through all the nervous layers except Jacob's membrane. They commence on 

 the inner surface of the retina by a conical base, the edges of the bases of adjoining 

 fibres being united, and thus forming a boundarv-line which is the membrana 

 limitans interna. As they pass through the various lavers they present a roughness 

 on their surface, as if a number of membranous processes had been abruptly 

 broken off. By these they are continuous with the reticulum of the inner and outer 

 molecular layer and with a sponge-like stroma, in which the nuclei of the inner 

 nuclear lavers are imbedded. In the inner nuclear laver each fibre of Miiller 



