THE EYE AS AN OPTICAL INSTRUMENT. 515 



and give off lateral branches, which are especially numerous 

 in the molecular layers. Like the limiting membranes they 

 are merely supporting tissues. 



On account ol the way in which the supporting and essen- 

 tial parts are interwoven in the retina it is not easy to track 

 the latter through it. There is, however (Chap. XXXIII), 

 good evidence that light first acts upon the rod and cone 

 layer, traversing all the thickness of inner strata of the retina 

 to reach it, before starting those changes which result in 

 visual sensations; and it is therefore probable that the rods 

 and cones are in direct continuity with the optic nerve-fibres. 

 The limiting membranes, with the fibres of Miiller and their 

 branches, are undoubtedly merely accessory and supporting. 



Each rod and cone consists of an outer and an inner seg- 

 ment. The outer segments of both tend to split up trans- 

 versely into disks and are very similar, except that those of 

 the rods are longer than those of the cones and do not taper 

 as the latter do. Moreover, the visual purple is entirely con- 

 fined to the outer segments of the rods, the cones containing 

 none of it. The inner segments of the cones are swollen, 

 while those of the rods are narrow and nearly cylindrical. 

 Over most of the retina the rods are longer and much more 

 numerous than the cones, but near the ciliary processes they 

 cease before the cones do; and in the yellow spot elongated 

 cones alone are found. In this region the whole retina is 

 modified; at its margin all the layers are thickened but 

 especially the nerve-cell layer, which becomes six or seven 

 thick, while elsewhere the cells are found in but one or two 

 strata. Most of the fibres run obliquely, reaching in to become 

 continuous with the cones of the central pit, which are long, 

 slender, and very closely packed. In the fovea itself all the 

 layers, except that ot the cones, thin away, and thus the depres- 

 sion is produced. The fovea is the seat of most acute vision; 

 when we look at an object we always turn our eyes so that the 

 light proceeding from it shall be focussed on the two foveae. 

 Where the optic nerve enters, all the layers but the nerve- 

 fibre layer (which is there very thick), and the internal limit- 

 ing membrane, are absent. 



The blood-vessels of the retina lie almost entirely in the 

 nerve-fibre and nerve-cell layers. 



The Refracting Media of the Eye are, in succession from 

 before back, the cornea, the aqueous humor, the crystalline 

 lens, and the vitreous humor. 



