THE STRUCTURE AND FUNCTIONS OF THE EYE, 253 



the sliglitest pressure so deranges it, as to lessen or destroy its 

 transparency. 



The crystalline humour may be considered as composed of 

 numerous concavo-convex lenses, admirably fitted to each other ; 

 those of the largest size having their circumference or edge op- 

 posed and nicely joined to each other, thus forming altogether a 

 double convex lens. The point of union between the two largest 

 lenses is embraced by a band of muscular fibres, disposed in a 

 circular direction, and named the ciliary ligament. These, by 

 contracting, assisted probably by the muscles of the eye, increase 

 the convexity of the crystalline lens, or rather of the series of 

 concavo-convex lenses of which the eye is composed, in a manner 

 so just and equal, as to adapt it to the distance of the object at 

 which the animal is looking : while, by the relaxation of these 

 muscular fibres, the convexity of the lens is diminislied by its 

 own elasticity. These changes take place with inconceivable ra- 

 pidity and accuracy. There are other muscular fibres proceeding 

 from the band of circular fibres named ciliary processes in a 

 straight, or rather radiated direction, towards the second coat of 

 the eye, named tunica choroides ; these are so arranged as to be 

 drawn into folds, by which disposition they are enabled to per- 

 form their office more easily, which is that of drawing the lens 

 towards the optic nerve, and thereby increasing the intensity of 

 vision, whereby the animal is enabled to see small objects dis- 

 tinctly. The crystalline lens is enclosed in a transparent capsule, 

 which is not in contact with it, there being about one drop, as it 

 is computed, of a transparent liquid interposed, which, from the 

 anatomist's name who first observed it, has been called " liquor 

 Moro;agnii."l 



I have seen a case where the convexity of the lens has been so 

 increased, by an unusual degree of contraction of the circular 

 fibres named ciliary ligament, that it burst the capsule, and was 

 forced out of .its situation. I found it lying with its posterior 

 convex surface on the inferior margin of the iris ; about half of 

 it appearing in the anterior chamber of the eye, as it is termed. 

 The transparency of the lens was not aflfected, at least it did not 

 appear so to me, and I rather think the accident had occurred 

 just at the time I observed it, from twitching the horse violently. 

 In cataract, a disorder in which the crystalline lens becomes 

 opaque, it always becomes globular in its form from an irritable 

 state of the band of circular fibres ; the same effect may be ob- 

 served in the circular fibres of the iris, causing the pupil to be 

 small, even in a moderate light, while the inflammation is going 

 on which occasions the cataract ; but when the opacity is such 

 as to exclude the light from the posterior part of the eye, which 

 contains the third or vitreous humour, with the choroid coat, 

 retina, and optic nerve, then the circular fibres relax, and the 



