120 ZOOLOGY. 



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 vas- 

 cular, 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 eye, in its way 

 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 

 rays of light into the interior of the eyeball; they change 

 the direction of the rays of light. The eye is but a kind of 

 camera obscura, the image of objects being as it were painted 

 on the retina; this image we see, and not the object itself. 



To understand this part of the history of vision, it is only 

 necessary to refer briefly to some of the laws of optics. 



Light travels in straight diverging lines. When they fall 



