As the term cataract is usually understood, it includes changes in the crystalline lens, which convert it from a transparent body into an opaque mass (fig. 256), which may be compared to a small biconvex piece of chalk; the appearance, in fact, is the same as that which is constantly noticed in the crystalline lens of fishes when cooked, and seen in the dish as a small globular body of a dense white colour. The professional man, however, recognizes the fact that the term cataract applies to any opaque speck, or number of specks, however minute they may be, which appear in any part of the lens or its capsule; indeed, a cataract may be so extremely minute as to be absolutely invisible to the naked eye, and the use of an ophthalmoscope by an expert is indispensable for the detection of the disease in its initial stage.
One of the old veterinary writers, Dr. Bracken, in 1737, describes four varieties of cataract, i.e. white, pearl colour, yellow and black or green cataract. All these varieties are said to have been met with recently in different animals, but they are only to be detected on post-mortem examination, so far, at least, as the peculiarity of colour is concerned, and as a rule, post-mortem examinations are not common in connection with the existence of this disease. If the cataract is perfectly visible there is no particular object in making a special examination of the lens after the death of the animal, and if there is no evidence of its existence, it is not suspected, and therefore not sought for; but it must be clear that in all cases cataract at the commencement may be discovered by the use of optical instruments, and their use is undoubtedly called for in all cases where a horse is in the constant habit of shying, or in any way gives reason for a suspicion that the sight is defective.
Fig. 256, - Cataract Complete.
Besides the division of cataracts as to colour, there is another which affects the capsule of the lens, capsular cataract, and cataracts which exist in the substance of the lens, lenticular cataracts (fig. 257). Capsular cataracts usually appear on the front of the capsule, and in the form of small circles or extremely minute specks, the smallest of which are only rendered visible by the use of the ophthalmoscope. With this instrument the examiner has no difficulty in distinguishing capsular cataract by transmitting the rays of light obliquely into the eye so as to get a lateral view of the surface of the capsule.
From the smallest speck of opacity the disease may be said to grow, often in radiating lines, until at length the whole of the lens is implicated, and thus a number of separate spots, which are detectable in the early stages of the disease, become consolidated into one opaque mass.
A somewhat different series of changes occurs in the development of cataract by exudation, which seems to affect the whole of the lens at the same time, modifying its transparency very slightly in the first instance, causing merely a slight cloudiness which gradually becomes more marked, going on to what is described as a milky condition, and ending in the chalky state which is really complete opacity. It is somewhat remarkable that, to the eye of the examiner, the lens presents a milky appearance, when on post-mortem examination it has been found to present a marked amber tint without the least trace of whiteness in any part. The deposit, however, is sufficiently dense to render the body incapable of transmitting light. It is interesting to notice that the older writers had very much the same idea of the pathology of the disease as is entertained at present. They referred to the opacity of the crystalline humour, and mentioned the circumstance of the opaque spots affecting the capsule or some of the layers (lamellae) of the lens itself. They looked upon cold, injury, and, in particular, periodical attacks of inflammation of the eyes as the chief causes of cataract.
Fig. 257. - Capsular and Lenticular Cataracts.
a, Crystalline Lens. b, Capsule of the Lens. c, (Diagram A), Capsular Cataract. c, (Diagram B), Lenticular Cataract.
Bracken and Gibson, in 1737, and James White, in 1802, trace the disease to repeated attacks of inflammation, and by the terms of their description they leave no doubt in the mind of the reader that the disease to which they trace the origin of cataract is the periodic ophthalmia of later times. Particularly they note that when the cataract is fully formed the ophthalmic disease ceases to appear at the accustomed times.
Gibson remarks that the disease known as moon-blindness, which is one of the terms applied to periodic ophthalmia, was really nothing but the prognostic of the breeding of cataracts, and he adds that he does not remember to have met with a case, excepting in the eyes of a horse, which had been called moon-blindness. James White also speaks of heredity as a cause of moon-blindness, and that of the most intractable sort.
Notwithstanding the various causes from which cataract is said to result, there is no doubt that it sometimes appears without the animal giving any evidence of active disease. The writer has known several instances of this kind, where cataract developed without any obvious cause and without any symptoms of inflammation being presented.
The question of treatment has always given rise to a good deal of difference of opinion. In the time of Drs. Bracken and Gibson the operation of couching, as it was called, was very well known to them, and they agree that the disease cannot be cured by any outward application of remedies, but only by a surgical operation, which they admit is not applicable to the lower animals on account of the impossibility of adjusting an artificial lens to compensate for the loss of the actual one. In this respect, however, the modern oculist may claim some little advance. Dr. Randolph of the Johns Hopkins hospital published some time ago an account of the removal of two cataracts in a dog. The writer comments on the general belief that artificial lenses are absolutely necessary, and goes on to state that, according to his own experience, their use has been much overrated, and he quotes cases of his patients who obtained, after some considerable practice, a fair degree of vision without the use of spectacles. In support of his opinion he mentions cases of a reproduction of the lens in perfect form after complete removal. Dr. Randolph's own operations on dogs suggest the possibility of restoring a certain degree of visual power to animals which are perfectly blind. In the case of the dog the operation of the removal of the lens was performed under chloroform on one eye, and in three weeks the sight was so far restored that the clog was able to make his way rapidly through a passage made by placing chairs for the purpose, and to jump over a chair which was placed in a doorway. The operation on the second eye was equally satisfactory, and after recovery the animal was used by his owner with other pointers in the field, and was reported to be quite equal to them in his work.
Dr. Randolph ascertained that several similar cases had been reported in foreign journals. In one case an ass was the subject, with what result is not known; but the fact of the restoration of the lens after removal in dogs, rabbits, and cats renders it at least probable that the same thing-might happen in the case of the horse, and the experiment is worth a fair trial, especially when the disease is detected in the very early stage.