Tl6 ESSENTIALS OF PHYSIOLOGY. 



of the cornea. The anterior surface of the iris is covered by a single 

 layer of flattened cells, continuous over the ligarnentum pectinatum 

 with that of the posterior surface of the cornea. The posterior surface 

 is covered by a double layer of cubical pigment cells continuous with 

 the pars ciliaris retinae. A ring of circularly arranged smooth muscle 

 fibres lies close to the margin of the pupil, forming the sphincter 

 pupillae, and radially arranged smooth muscle fibres, the dilator pupil! ae, 

 lie close to the posterior surface of the iris. 



The internal coat of the eye consists of the retina, a delicate, semi- 

 transparent membrane lining the posterior three-fourths of the eyeball, 

 and ending abruptly just behind the ciliary processes in a jagged 

 margin, the ora serrata. The retina consists from without inwards of 

 the following layers (fig. 33) : 



1. The layer of pigment cells. 



2. The layer of rods and cones. 



3. The outer nuclear layer. 



4. The outer molecular layer (outer plexiform layer). 



5. The inner nuclear layer. 



6. The inner molecular layer (inner plexiform layer). 



7. The layer of ganglionic nerve cells. 



8. The layer of nerve fibres (stratum opticum). 



The structures forming these layers are supported by the fibres of 

 Miiller, which extend from the level of the bases of the rods and cones 

 to the inner surface of the retina. The ends of Mliller's fibres are 

 expanded and fused together to form the outer and inner limiting 

 membranes, the former lying between the layer of rods and cones and 

 the outer nuclear layer, and the latter bounding the retina internally. 

 Each fibre has a nucleus at the level of the inner nuclear layer. 



(1) The cells of the pigment layer are hexagonal on surface view, 

 and when seen from the side they exhibit an outer non-pigmented 

 portion containing a nucleus, and an inner pigmented part from which 

 delicate processes run between the rods and cones. 



(2) and (3) The layer of rods and cones and the outer nuclear layer 

 together form one layer of neurons. Each rod consists of (a) an outer 

 cylindrical segment which is transversely striated, and which, in a dark- 

 adapted retina, contains visual purple or rhodopsin, and (6) an inner 

 fusiform segment, vertically striated in its outer fourth and granular 

 in the remaining three-fourths. Each rod is prolonged into the outer 

 nuclear layer as a varicose fibril, in the course of which is a nucleus. 

 The nucleus in some animals is transversely striated. Each fibril ends 

 in a knob in the outer molecular layer, the knobs of several adjacent 

 rod neurons coming into relationship with the dendritic arborisation of 



