718 



THE EYE. 



with chiefly a longitudinal direction ; and tbe ramified cells, fewer in 

 number, are devoid of pigment towards the free extremities of the folds. 



The pigmentary layer (choroidal epithelium, membrane of the black pig- 

 ment) forms a thin dark lining to the whole inner surface of the choroid and 



Fig. 464. PIGMENT CELLS OP THE MIDDLE COAT 

 (after Kolliker). 



A, small portion of the chorcid with the stellate 

 or ramified cells which form its stroma. B, pigment 

 cells, which cover the inner surface of the choroid ; 

 a, these cells seen from the surface, of hexagonal 

 form, and showing nuclei in their interior ; b, three 

 of the same cells viewed edgeways ; c, molecular 

 pigment, which fills the cells. 



the iris. As far forwards as the ciliary pro- 

 cesses it consists of only a single layer of 

 flat six-sided cells, applied edge to edge liko 

 mosaic work. Each cell contains a nucleus 

 and more or less dense molecular contents, 

 accumulated in greatest abundance towards 

 the circumference of the cell, and parrly 

 obscuring the nucleus. On the ciliary pro- 

 cesses and the iris the pigment is several lay- 

 ers deep, and the cells, smaller and rounded, 

 are so filled with dark pigment as to cover 

 up the nucleus. In the eye of the albino, 



pigment is absent both from the hexagonal cells and the ramified corpuscles 



of the choroidal tunic. 



It may be mentioned that in fishes, and in many mammals, including the ox and 

 the sheep, the eyes of which are often selected for dissection, the choroid, instead of 

 being uniformly lined with dark pigment, presents on a greater or less extent of its 

 back part a silvery layer named tapetum. The tapetum in ruminants consists of 

 tendinous fibres, and in carnivora and fishes of cells, filled, in the carnivora, with 

 granular matter (Leidig), in fishes with slender rods. On its inner surface is the 

 tunic of Ruysch, as well as the layer of hexagonal cells, which, however, is here 

 destitute of pigment. 



THE IRIS. 



The iris is the contractile and coloured membrane which is seen behind 

 the transparent cornea, and gives the tint to the eye. In its centre it is 

 perforated by an aperture the pupil. 



By its circumferential border, which is nearly circular, the iris is connected 

 with the choroid, the cornea, and the ciliary ligament and muscles : the free 

 inner edge is the boundary of the pupil, and is constantly altering its dimen- 

 sions during life. The iris measures J an inch across, and, in a state of 

 rest, from the circumference to the pupil about ^-th of an inch. Its surfaces 

 look forwards and backwards. The anterior, variously coloured in different 

 eyes, is marked by waving lines converging towards the pupil, near which 

 they join in a series of irregular elevations ; and, internal to these, other 

 finer lines pass to the pupil. The posterior surface is covered with dark 

 pigment ; and this being removed, there is seen at the margin of the pupil a 

 narrow circular band of fibres (sphincter muscle of the pupil), with which 

 lines radiating inwards are blended. 



