VISION. 561 
tudinal section. Other specimens may be dissected with and 
without the use of water. . 
1 
Fie. 404.—Certain parts of eye. 1x10. (After Sappey.) 1,1, crystalline lens; 2, hyaloid 
membrane ; 8, zonule of Zinn; 4, iris; 5, a ciliary process ; 6, radiating fibers of ciliary 
muscle ; 7, section of circular portion of ciliary muscle ; 8, venous plexus of ciliary mus- 
cle; 9,10, sclerotic coat; 11,12, cornea; 13, epithelial layer of cornea; 14, Descemet’s 
membrane ; 15, pectinate ligament of iris ; 16, epithelium of membrane of Descemet ; 17, 
‘union of sclerotic coat with cornea ; 18, section of canal of Schlemm. 
Assuming that some such work has been done, and that the 
student has become quite familiar with the general structure 
of the eye, we call attention specially to the strength of the 
sclerotic coat; the great vascularity of the choroid coat and its 
terminal ciliary processes, its pigmented character adapting it 
for the absorption of light; the complicated structure and pro- 
tected position of the retinal expansion. It may be said that 
the whole eye exists for the retina, and that the entire mechan- 
ism besides is subordinated to the formation of images on this 
nervous expansion. The eye of the mammal may be regarded 
as an arrangement of refracting media, protected by coverings, 
with a window for the admission of light, a curtain regulating 
the quantity admitted; a sensitive screen on which the images 
are thrown; surfaces for the absorption of superfluous light; 
86 
