HUMORS OF THE EYE :—CONJUNCTIVA. 419 
certain circumstances tlie image that falls upon it may not be 
perceived at all (§ 554:). 
536. The cavity of the globe is occupied by three humors 
of different consistence—the Aqueous, Vitreous , and Crystal¬ 
line. The aqueous humor is nearly pure water, being nothing 
else than the serum of the blood very much diluted : it occu¬ 
pies the anterior chamber ca, and a small space behind the 
iris, in front of the crystalline lens. The vitreous humor v 
resembles thin jelly in consistence, and occupies the greater 
part of the globe of the eye behind the iris. The crystal¬ 
line humor cr is of much firmer consistence, resembling 
very thick jelly or soft gristle; it has the form of a double- 
convex lens, the posterior surface of which is more convex 
than its anterior; and hence it is commonly known as the 
crystalline lens. It is suspended in its place by a set of little 
bands pc, proceeding from the choroid coat, and known as 
the ciliary processes. 
537. The cornea is covered externally by a membrane bb , 
termed the Conjunctiva. This membrane is perfectly transparent 
where it covers the cornea, and seems like an outer layer of 
it; the front of the sclerotic also is covered by it, but it is 
there semi-opaque, having a whitish colour. The membrane 
does not pass back over the globe of the eye, however, but 
bends forward again, as seen at bb, so as to form the lining of 
the eyelids, at the edge of which it becomes continuous with 
the skin. Thus the smooth surfaces of the eye and of the 
under side of the lids are both formed by this membrane ; 
the mucous secretion from which serves to diminish the 
friction of one upon the other. But the smallest particle of 
any hard substance, which may become interposed between 
these surfaces, produces great irritation. It cannot pass far 
backwards, however, on account of the bend of the membrane 
at bb ; and thus it may be always removed (if loose) with 
little difficulty. The lower lid can be easily drawn down, so 
as to expose the membrane as far as this bend; and any loose 
particle that is lying upon its surface may thus be detected 
and removed. But the upper lid, being longer, cannot be 
drawn out sufficiently for this purpose; and it is necessary to 
evert it, or turn it inside-out. As the knowledge of the mode 
of performing this very simple operation will often save a 
good deal of suffering, it will be here described. Nothing 
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