46 



THE EYE. 



The layers hitherto described contain structures (cells and fibres) which are undoubtedly 

 of nervous nature, and which appear to be developed in the same manner as corresponding- 

 structures in the brain. Those next to be described are of epithelial nature, and constitute 

 collectively what is sometimes known as the cyitJieliiim of tlic rttiiui. in contradistinction to 

 the more strictly nervous or cerebral part. The outer nuclear and bacillary layers are 

 morphologically but one, being composed of long cells, visual cells, which extend through 

 both layers. Each cell is drawn out into a fibre, and furnished with a nucleus in its inner 

 portion (rod- or cone- fibre and its outer granule), and is peculiarly modified both in shape and 

 structure in its external portion (rod or cone proper). 



In most vertebrates no blood-vessels penetrate into this epithelial layer, but a remarkable 

 exception is stated by Denissenko to occur in the retina of the eel, in which the retinal 

 capillaries extend nearly to the limitans externa. 



6. Outer nuclear layer. This (figs. 48, 50, 52) resembles very closely at first 

 sight, in sections of retina stained with hasmatoxylin or carmine, the inner nuclear 



Fig. 52. DIAGRAM OF SOME OF THE NERVOUS AND EPITHELIAL 

 ELEMENTS OF THE RETINA. (Modified from Schwalbe. ) 



The numbers are the same as in fig. 47. 



layer ; appearing, like that, to consist of clear, oval or 

 elliptical, nuclear corpuscles (outer granules), from the 

 ends of which delicate fibres are prolonged. These 

 outer granules differ, however, essentially from the 

 inner granules, and may be readily distinguished from 

 them. They are of two kinds, which present well- 

 marked differences, and are known respectively as the 

 rod-granules and cone-granules, accordingly as they 

 are connected with the rods or with the cones of the 

 next retinal layer. Those which are connected with 

 the rods are, in most parts of the retina, by far the 

 more numerous, and form the main thickness of the 

 outer nuclear layer. They may be regarded as enlarge- 

 ments or swellings in the course of delicate fibres (rod- 

 fibres), which extend from the inner ends of the rods 

 at the membrana limitans externa through the thick- 

 ness of this layer to the outer molecular layer. The 

 enlargements, of which there is but one to a fibre, 

 situated at any part of its course, are each occupied by 

 an elliptical nucleus, and, in the fresh condition, 

 exhibit a remarkable cross-striped appearance (Henle), 

 the strongly refracting substance which mainly com- 

 poses the enlargement being interrupted by bands or 



disks of a clearer, less refracting material, usually two in number, one on each side of 

 the middle line (fig. 52), but occasionally single and median (see the left-hand one 

 in fig. 52). The rod-fibres are of extreme fineness, and exhibit minute varicosities 

 in their course : each is directly continuous at the outer end with one of the rods, 

 but at the inner end terminate in a somewhat larger varicosity (end-knob). These 

 end-knobs lie in the outer part of the outer molecular layer, and are embedded in 

 the tufts of which the terminal arborisations of the rod-bipolars are formed. In 

 amphibia and birds (bat not in nocturnal birds) fine fibrils radiate from these 

 end-knobs (fig. 51), but in mammals and teleosteans these fibrils are absent 

 (Cajal). 



Those outer granules which are connected with the cones are, in most parts of 

 the retina, much fewer in number than the rod-granules, from which they are dis- 



