SENSITIVE SYSTEM. 391 



Crystalline Lens. 



The crystalline lens, though a solid body, is always 

 considered as one of the humours. It is named crystalline from 

 its resemblance to crystal, and lens from its shape. It is 

 lodged between the aqueous and vitreous humours, the latter pre- 

 senting in front a hollow bed for its reception ; and it is surrounded 

 by the ciliary processes, and parted from the iris by the posterior 

 chamber. The lens is inclosed within a capsule of its own, by 

 whose attachment to the tunica vitrea it is retained in its place, 

 with the assistance of the membranula corona ciliaris. This cap- 

 sule, the tunica crystallina of some, is denser, firmer, and more 

 resisting than the tunica vitrea : it cannot be shewn to have any 

 adhesion to the lens itself, there being between them an aqueous 

 moisture, which some regard merely as a post-mortem exudation, 

 while others consider it as necessary during life to prevent adhesion 

 of the contiguous surfaces, and call it the aqua vel liquor Mor- 

 gagni, from its discoverer. The anterior part of the capsule is 

 thicker, stronger, and more elastic than the posterior part ; 

 though the latter is something strengthened by the membranula 

 ciliaris, and is closely, but not inseparably, united by a fine 

 cellular web to (and consequently cannot, as some have supposed, 

 be continuous in substance with) the tunica vitrea. Successful 

 injections shew the vascularity of the capsule ; and the liquor 

 Morgagni is supposed to be a secretion from its vessels, which 

 themselves are derived from the central artery of the retina. 



The lens in figure approaches more or less to a sphere : it is not 

 uniformly spherical, but is composed of the segments of two un- 

 equal spheies, the posterior of which has more convexity than 

 the anterior. Though perfectly transparent, it has this peculia- 

 rity in its composition — that it is soft externally, but gradually 

 increases in firmness and density of substance from the super- 

 ficial to the central parts, which latter becomes its nucleus. It 

 appears to be placed, as it were, in a state of insulation within the 

 capsule ; for we cannot detect any cellular, vascular, or other direct 

 connexion between one and the other; and we constantly find 

 both their surfaces moist with the liquor Morgagni, rendering 

 them so slippery that whenever the capsule is wounded the 

 lens readily makes its escape. — The nature and composition of the 

 lens is a subject still open to inquiry. As in other doubtfid and 

 obscure cases, where anatomical tests forsake us, hypothesis is 

 ready to supply their place, and, as in the present instance, now 

 and then furnishes us with some ingenious and plausible con- 

 jectures. Dr. Young* thought that at one time he saw in the 



* 'S'ounj" " Oh Nataiul Pliilosopliif." 



