780 



SPECIAL SENSES. 



Crystalline Lens. The crystalline is a double-convex lens, transparent, and exceed- 

 ingly elastic. It has a function in the refraction of the rays of light analogous to the ac- 

 tion of convex lenses in optical instruments. When we come to study its exact structure, 

 however, we shall find many points that are still undetermined and somewhat obscure ; 

 but, fortunately, these are not, as far as we know, of much physiological importance. 



The lens is situated behind the pupil, in what is called the hyaloid fossa of the vitre- 

 ous humor, which is exactly moulded to its posterior convexity. In the foetus, the cap- 

 sule of the lens receives a branch from the arteria centralis, but it is non-vascular in the 

 adult. The anterior convexity of the lens is just behind the iris, and its borders are in 

 relation with what is known as the suspensory ligament. The convexities do not present 

 regular curves, and they are so subject to variations after death that the measurements, 

 post mortem, are of little value. During life, however, they have been measured very 

 exactly in the various conditions of accommodation. The diameters of the lens in the 

 adult are about ^ of an inch transversely and ^ of an inch antero-posteriorly. The con- 

 vexity is greater on its posterior than on its anterior surface. In foetal life, the convexi- 



FIG. 24T. Crystalline lens anterior view. (Babuchin.) 



ties of the lens are much greater than in the adult and its structure is much softer. In 

 old age, the convexities are diminished and the lens becomes harder and less elastic. 

 The substance of the lens is made up of layers of fibres of different degrees of density, 

 and the whole is enveloped in a delicate membrane, called the capsule. 



The capsule of the lens is an exceedingly thin, transparent membrane which is very 

 elastic. This membrane is generally from -^^ to y^ of an inch thick; but it is very 

 thin at the periphery, measuring here only -^^ of an inch. Its thickness is increased in 

 old age. On the anterior portion, the capsule is lined on its inner surface with a layer 

 of exceedingly delicate, nucleated epithelial cells. The posterior half of the capsule has 

 no epithelial lining. The cells are regularly polygonal, measuring from -gfaf to T1? Vff 

 of an inch in diameter, with large, round nuclei. After death they are said to break 

 down into a liquid, known as the liquid of Morgagni, though by some this liquid is sup- 

 posed to be exuded from the substance of the lens. At all events, the cells disappear 

 soon after death. 



If the lens be viewed entire with a low magnifying power, it presents, upon either of 



