THE EYE. 179 



creatures. For example, M. Chossat found that the cornea of the 

 eye of an ox was an ellipsoid of revolution round the greater axis, 

 this axis being inclined inwards about io°. The ratio of the 

 major axis to the distance between the foci in the generating 

 ellipse he found to be i '3 ; and this agreeing very nearly with 

 1*337, the index of refraction of the aqueous humour, it follows 

 from the laws of light, that parallel rays will be refracted to a focus 

 by the surface of this humour with perfect accuracy. 



He also found that the two surfaces of the crystalline lens are 

 ellipsoids of revolution round the lesser axis ; and it is somewhat 

 remarkable that the axes of these surfaces do not coincide in 

 direction with each other, or with the axis of the cornea ; these 

 axes being both inclined outwards, and containing with each other, 

 in the horizontal section in which they lie, an angle of about 5*^. 

 The same author found that the cornea of an elephant's eye is an 

 hyperboloid.* 



The cornea consists of five layers : — Conjunctiva ; Anterior 

 elastic layer ; Fibrous layer, arranged in layers ; Posterior elastic 

 layer; Endothelium. 



The Choroid, Fig. 3 (I?), is the vascular tunic of the eye, and 

 is stained in man with a deep brown or black pigment. The 

 outer surface is flocculent through the attachment of the cellular 

 tissue uniting it with the sclerotic. The inner surface is smooth, 

 and is covered by a layer of densely pigmented, and more or less 

 hexagonal cells, beneath which it is highly and minutely vascular. 

 The substance of the choroid is mainly formed of the ramifica- 

 tions of the arteries and veins, and is traversed by the ciliary 

 nerves on their way to the I'n's. 



On the outer and anterior border of the choroid is a circle of 

 grey, softish substance, mainly the ciliary muscle, applied like a 

 band round the margin of the aperture, into which the iris is 

 fitted. It adheres closely to the sclerotic at the line of the junc- 

 tion of the cornea. This zone is called the ciliary ligament {n). 

 On the inner border of the choroid is a circle of longitudinal 

 folds of that membrane, called ciliary processes [k). The free 

 central or internal border of each fold enters into the contiguous 



* Lloyd on "Light and Vision." 



