440 CONJUNCTIVA AND SCLEROTIC. 



The skin on the outer surface is a continuation of that of 

 the face, and passes directly into the mucous membrane at 

 the free border of the lid. This last covers the inner surface 

 of the lid as far as to the osseous margin of the orbit, where it 

 is reflected upon the eyeball, on which its anterior segment 

 may be traced as far as to the margin of the cornea. 



That portion of the membrane which lines the IVls is termed 

 the conjunctiva palpebrarum ; the* point of reflection is named 

 the fornix conjunctive ; and lastly, the portion covering the eye 

 is called the conjunctiva bulbi. At the inner angle of the eye 

 the conjunctiva bulbi forms a fold, the plica semiiunaris, which 

 is considered to be the rudiment of a third lid, or membrana 

 nictitans. Heinrich Miiller found smooth muscular fibres in 

 this fold, and these may also consequently be regarded as a 

 rudiment of the muscle of the membrana nictitans. 



In the domestic animals Ley dig,* and in the elephant 

 Harrison, has observed tjie presence of dense plates composed 

 of true cartilage. 



In the Batrachia the membrana nictitans is specially cha- 

 racterised by its structure and its optical relations. When 

 fresh (living), it is so transparent that after excision, on placing 

 it in the aqueous humour or in the serum of blood, it may be 

 examined with the highest powers ; and if the somewhat 

 thicker margins are cut away, a perfectly plane expansion 

 is obtained, admirably adapted for examination with high 

 powers. 



In such specimens the opportunity is afforded of investigating, 

 whilst perfectly fresh, epithelium, connective tissue, bloodvessels, 

 nerves, and glands. Above all, the bloodvessels are here brought 

 into view more beautifully than is the case with any other 

 known organ of the adult animal after removal from the living 

 body. The opportunity is also afforded of examining the simple 

 flask-like glands, and at all depths their excretory ducts as they 

 perforate the external epithelium. Lastly, medullated nerve 

 fibres may be examined, either singly or united into fasciculi, 

 in regard to which it may be said that they at least approximate 

 very closely to their living condition. 



* Lehrbuch, der Histologie, 1857. 



