388 STRUCTURE OF VERTEBRATES. 



the posterior part of the globe is the sensitive retina, inner- 

 vated by fine branches from the optic nerve. It may be com- 

 pared to the nervous matter of the brain, from which, indeed, 

 it arises. Outside of the retina is a pigmented epithelium, 

 and outside of this a vascular membrane ; together these are 

 often called tHe choroid. The vascular part may be com- 

 pared to the pia mater of 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 of the brain, and also mesoblastic in origin. Occupying 

 the front of the globe is the crystalline 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 conjunctiva — 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 alter- 

 able by the action of accommodating muscles arranged 

 in a circle at the junction of iris and sclerotic. In many 

 Fishes and Reptiles, and in Birds, a vascular fold called the 

 pecten projects from the back of the eye into the vitreous 

 humour. The retina is a very complex structure with several 

 layers of cells, partly supporting and partly nervous, but 

 suffice it to say that the layer next the vitreous humour 

 consists of nerve-fibres, while that furthest 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, 

 while 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. 



Among the extrinsic structures, must be noted the six 

 muscles which move the eye-ball, the upper and lower eye- 

 lids which are often very slightly developed, the third eyelid 

 or nictitating membrane. Above Fishes there is a lachrymal 

 gland associated with the upper lid, and a Harderian gland 



