EXTERNAL STRUCTURE. "95 



convex on the inner than the outer side. It is enclosed in a delicate trans- 

 parent bag or capmle, and is placed between the aqueous and the vitreous 

 humors, and received into a hollow in the vitreous humor with which it 

 exactly corresponds. It has, from its density, and its double convexity, 

 the chief concern in conveying the rays of light which pass into the pupil. 



The lens is very apt to be affected from long or violent inflammation of 

 the conjunctiva, and either its capsule becomes cloudy, and imperfectly 

 transmits the light, or the substance of the lens becomes opaque. The 

 examination of the horse, with a view to detect this, must either be in the 

 shade, or at a stable door, where the hght shall fall on the horse from 

 above and in front ; and in conducting this examination we would more 

 particularly caution the intended purchaser against a superfluity of white 

 about his neck. Holding the head of the horse a little up, and the light 

 coming in the direction which we have described, the condition of the lens 

 will at once be evident. The confirmed cataract, or the opaque lens of 

 long standing, will exhibit a pearly appearance, which cannot be mistaken, 

 and will frequently be attended with a change of form, a portion of the lens 

 being forced forwards into the pupil. Although the disease may not have 

 proceeded so far as this, yet if there be the slightest cloudiness of the lens, 

 either generally, or in the form of a minute spot in the centre, and with or 

 without lines radiating from that spot, the horse is to be condemned ; for in 

 ninety-nine cases out of a hundred the disease will proceed, and cataract, 

 or complete opacity of the lens, and absolute blindness, will be the result. 



Cataract in the human being may, to a very considerable extent, be 

 remedied. The opaque lens may be extracted, or it may be forced into 

 the vitreous humors, and there existing as a foreign body it will soon 

 be absorbed and disappear. These operations are impossible in the horse, 

 for, in the first place, there is a muscle of which we have already spoken, 

 and to be presently described, peculiar to quadrupeds, and of such power 

 as generally to draw back the eye too far into its socket for the surgeon 

 to be enabled to make his incision ; and, could the incision be made, the 

 action of this muscle would force out the greater part of the contents of 

 the eye, and this organ would almost waste away. If, however, the opaque 

 lens could be withdrawn or depressed, and the mechanism of the eye were 

 not otherwise injured, the operation would be totally useless, for we could 

 not make the horse wear those spectacles, whose converging power might 

 compensate for the loss of the lens. 



Behind the lens, and occupying four-fifths of the cavity of the eye, is the 

 vitreous humor (glassy, or resembling glass). It seems, when first taken 

 from the eye, to be of the consistence of a jelly, of beautiful transparency ; 

 but if it is punctured, a fluid escapes from it as limpid and as thin as water, 

 and when this has been suffered completely to ooze out, a mass of mem- 

 braneous bags or cells remains. The vitreous humor then consists of a 

 watery fluid contained in these cells, but the fluid and the cells form a body 

 of considerably greater density than the aqueous fluid in the front of the eye. 



Last of all, between the vitreous humor and the choroid coat, is the 

 retina, o, or net-like membrane. It is an expansion of the substance, g, 

 of the optic nerve. When that nerve has reached the back of the eye, and 

 penetrated through the sclerotic and choroid coats, it first enlarges into a 

 little w hite prominence, and from that proceed radiations or expansions 

 of nervous matter, which spread over the whole of the choroid coat, and 

 form the third investment of the eye. The membrane by which this nervous 

 pulp is supported, is so exceedingly fine and delicate, that it will tear 

 with the slightest touch, and break even with its own weight. The mem- 



