552 STRUCTURE OF VERTEBRATA 



The adult eye is more or less globular, and its walls con- 

 sist of several distinct layers. The innermost layer bound- 

 ing the posterior part of the globe is the sensitive retina, 

 innervated by fine branches from the optic nerve. It may 

 be compared to the nervous matter of the brain, from 

 which, indeed, it arises. Outside of the retina is a pig- 

 mented epithelium, and outside of this a vascular mem- 

 brane ; together these are often called the choroid. The 

 vascular part may be compared to the pia mater covering 

 the brain, and like it is derived from mesoblast. Outside of 

 the choroid is a protective layer or sclerotic, comparable to, 

 and continuous with, the dura mater covering the brain, 

 and also mesodermic in origin. Occupying the front of 

 the globe is the crystaUine lens, a clear ball derived directly 

 from the skin. It is fringed in front by a pigmented and 

 muscular ring — the iris, which is for the most part a 

 continuation of the choroid. The space enclosed by the 

 iris in front of the lens is called the pupil. Protecting and 

 closing the front of the eye is the firm cornea continuous 

 with the sclerotic, and covered externally by the con- 

 junctiva — -a delicate epithelium continuous with the 

 epidermis. Between the cornea and the iris is a lymph 

 space containing aqueous humour, while the inner chamber 

 behind the lens contains a clear jelly — the vitreous humour. 

 The lens is moored by '* ciliary processes " of the choroid, 

 and its shape is alterable by the action of accommodating 

 ciliary muscles arranged in a circle at the junction of iris 

 and sclerotic. In many Reptiles, and in Birds, a vascular 

 fold, called the pecten, projects from the back of the eye 

 into the vitreous humour. A similar fold in Fishes 

 {processus falcifortnis) ends in a knot-like structure in the 

 lens. It acts as an *' accommodator." The retina is a very 

 complex structure, with several layers of cells, partly 

 supporting and partly nervous ; the layer next the vitreous 

 humour consists of nerve-fibres, while that farthest from 

 the rays of light and next the pigment epithelium consists 

 of sensitive rods and cones. The region where the optic 

 nerve enters, and whence the fibres spread, is called the 

 blind spot, and near this there lies the most sensitive region 

 — ^the yellow spot, with its fovea centralis, where all the 

 layers of the retina have thinned off except the cones. 



