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 
(processus falciformis) ends ina knot-like structure in the lens. 
Itacts as an “‘accommodator.” The retina isa 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 
32 
