HUMOES OF THE EYE : CONJUNCTIVA. 419 



certain circumstances the 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 Aqu^pus, 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 j 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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