MEMBRANES OF THE EYE. 335 



From the preceding description we see that the air-tubes 

 and lungs are composed of cartilage, fibrous membrane, and 

 mucous membrane internally, and a serous^membrane exter- 

 nally ; besides blood-vessels and nerves. The cartilages when 

 long boiled in water mostly dissolve, but the solution does 

 not gelatinize, however much it may be concentrated. The 

 serous and mucous coats are doubtless of the [same nature 

 as those in other parts of the body, and the same must be the 

 case with the arteries, veins, and nerves. But no experiments 

 have been made upon the fibrous membrane ; though in exter- 

 nal appearance it bears considerable resemblance to the fibrous 

 coat of the arteries. Neither has anything been ascertained re- 

 specting the chemical nature of the tissue which connects the in- 

 numerable vesicles of which the lungs are composed with one 

 another. The lungs themselves have a peculiar appearance, dif- 

 fering from that of every other part of the body, and this must be 

 owing to the nature of the tissue which connects these vesicles to- 

 gether. But it would be extremely difficult, if not impossible, 

 to examine that tissue separate from the various membranes and 

 blood-vessels with which it is so intimately connected. 



CHAPTER XXVIII. 



OF THE MEMBRANES OF THE EYE. 



THE eye is a globular body filled internally with the aqueous 

 humor, the lens and the vitreous humor, and surrounded exter- 

 nally by three or four different membranes or coats. These are 

 the conjunctiva, the sclerotic coat, and the cornea ; the choroid 

 coat, Jacotfs membrane, and the retina. 



1. The conjunctiva lines the free border and inner surface of 

 the eyelids, from which it is reflected on the globe of the eye, so 

 as to cover its anterior third. It is red and vascular on the lids, 

 but firm and pale on the sclerotic, and very thin and transparent 

 on the cornea. The chemical properties of this coat have not 

 been ascertained ; but it is not unlikely that it has at least a great 

 analogy to the cuticle. 



2. The sclerotic may be considered as the true external coat 

 of the eye, since it covers the whole of it except the small por- 

 tion occupied by the cornea. It is thick, dense, and opaque. 



