390 SENSITIVE SYSTEM. 



it appears at first view merely to be an expanded mass of ner- 

 vous pidp ; but, by scra])ing the surface of it as it floats in water, 

 it becomes resolvable into a pulpy or medullary part (which is 

 seen dispersed in the fluid) and an extremely delicate membrane, 

 whose texture has been found to be almost entirely vascular; the 

 latter forming the basis or vascular network upon which the 

 former is spread, and by which it is nourished and supported. 

 This membrane is intimately united with the border of the open- 

 ing that gives passage to the optic nerve. It receives its vessels 

 from the central artery of the retina — a vessel that takes its 

 course through the axis of the nerve. 



OF THE HUMOURS. 



These are three in number, viz. the aqneous, crystalline, and vi- 

 treous ; and they occupy in succession the spaces in the anterior, 

 middle, and posterior parts of the globe. 



Aqueous Humour. 



The aqueous humour escapes as soon as the cornea is punc- 

 tured, and the cornea itself falls afterwards into wrinkles, shewing 

 that the convexity and tension of the one is owing to the presence 

 and pressure of the other. It fills the interval between the 

 cornea and crystalline lens, insulating the iris by which the 

 whole space is divided into two cavities, named the anterior and 

 posterior chambers: these chambers consequently communicate 

 through the pupil; but they are very unequal in their dimensions, 

 the posterior being nothing more than a very narrow chasm 

 between the uvea and the lens, surrounded by the ciliary pro- 

 cesses, whose points may be said to project into it. The aqueous 

 humour is a bright limpid fluid, and in its properties bears a great 

 resemblance to the vitreous, in the condition in which the latter 

 drops from its cellular case : they are both compounded of albu- 

 men, gelatine, and muriate of soda, suspended in a watery men- 

 struum. This humour (as well as the others) has a capsule of its 

 own; at least, we infer so from analogy, or rather from the separa- 

 bility of a thin transparent lamina from the concavity of the cornea 

 not possessing the fibrous character of the other corneal laminae, 

 and which some have been able, they say, to trace upon the iris; 

 though no one, I believe, has seen it enter the pupil : this mem- 

 brane is supposed to secrete the humour; and, if we may judge 

 from the rapidity with which the fluid is reproduced after it has 

 been let out, its secretion is by no means either a difficult or an 

 expensive process. 



