114 ZOOLOGY. 



the exterior ;uul tin- retina in the axis of vision, or measured 

 by the si/.e of tin- pupil. The cornea is everywhere abso- 

 lutely transparent, and so is the conjunctiva which covers it. 



In the iris, some of the fibres proceed like ra<lii from 

 the free edge of the pupil towards the base; others sur- 

 round, as it were, the pupil, and act like a sphincter in 

 contracting the orifice. The aqueous humour, though quite 

 transparent, contains a little albumen and salts, such as are 

 met with in all animal secretions. When suffered to escape 

 by a puncture or section of the cornea, it is speedily replaced. 

 The ciliary processes seen surrounding the capsule of the 

 lens, and which are appendages of the choroid membrane, are 

 extremely vascular, and may be the source of the secretion of 

 this humour and of the vitrine. 



The crystalline humour or lens is a body of considerable 

 density, composed of concentric layers. It is enveloped in a 

 distinct capsule, and when removed in young animals may be 

 replaced by another. The lens is more convex posteriorly 

 than anteriorly in man. 



The vitrine (vitreous humour) is a semi-solid body, enclosed 

 in a capsule, intersected by membranous partitions, in which 

 the fluid is contained. Its membrane is called the hyaloid. 



In albinos, the pigment, an appendage of the choroid and 

 iris, is wanting. 



Under a high microscope, the filaments of the optic nerve 

 seem to terminate in numerous cylindrical papillae, resembling 

 mosaic. 



232. Mechanism of Vision. The sun and bodies in a 

 state of ignition are visible in themselves ; but other bodies 

 are visible to us only by the reflection of light in such a way 

 as to reach us. 



Light moves with extreme rapidity; it affects us only 

 when it reaches the retina; opaque bodies reflect or absorb it; 

 transparent bodies, as air, offer it a free passage. 



Whatever obstructs the free passage of the light through 

 the conjunctiva, cornea, and humours of the e}'e, in its \\av 

 to the retina, obstructs or destroys vision ; hence the effects 

 of opacity of any of these structures : the cataract which 

 destroys vision is merely an opaque lens, which being removed 

 out of the axis of vision by the surgeon, restores the function 

 of the eye. 



But these diaphanous parts of the eye serve other purposes 

 besides the negative one of permitting the free passage of the 



