SENSE ORGANS. 497 



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 

 (processusfaldformis) 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. 



Among the extrinsic structures must be noted the six muscles which 

 move the eyeball, the upper and lower eyelids, which are often very 

 slightly developed, and the third eyelid or nictitating membrane. 

 Above Fishes there is a lachrymal gland associated with the upper lid, 

 and a Harderian gland associated with the nictitating membrane. In 

 Mammals there are also Meibomian glands. The secretions of all these 

 glands keep the surface of the eye moist. 



While the medullary groove is still open, the eyes arise 

 from the first vesicle of the brain as hollow outgrowths or 

 primary optic vesicles. Each grows till it reaches the skin, 

 which forms a thickened involution in front of it. This 

 afterwards becomes the compact lens. Meantime it sinks 

 inwards, and the optic vesicle becomes invaginated to form 

 a double-walled optic cup. The two walls fuse, and the 



