The Choroid.
It remains to add a few words to what has already been said about this membrane. The great majority of the changes we have found in it are, clearly, to be attributed to the effects either of hypertony or of hypotony of the eye; they do not, therefore, differ from similar appearances found under the same conditions generally.
Compared with detachments of the retina, those of the choroid are rare in this series. In one instance, already mentioned, the two membranes had been displaced together by the instrument used in couching (Pl. V., Fig. [30]). In another, in which severe inflammation had occurred, the retina and choroid had adhered to each other, and had been detached as one sheet by the contraction of an exudate, which lay in the neighbourhood of the iris and ciliary body and in the front part of the vitreous cavity. In a number of other globes, in which the tension was low, the ciliary body and the anterior portion of the choroid were found separated from the sclera as far back as the line where the anterior segment of the latter coat was drawn over the posterior in the manner pointed out by Treacher Collins in his work on Hypotony (Trans. of the O.S. of the U.K., 1917).
In the previous chapter we have spoken of an appearance commonly seen in successfully couched eyes—viz., an unusual distinctness of the large vessels of the choroid in the ophthalmoscopic picture. It is necessary to insist that in such cases the vessels are not sclerosed; they are seen with unusual distinctness simply because the pigment which usually hides them from view has disappeared. What is more, a careful study of a number of these cases has created a strong impression that the deficiency in pigmentation affects both the pigmentary layer of the retina and also the pigment of the choroid. The absence of the former lays bare what lies behind it; the absence of the latter is inferred from the general appearance of pseudo-albinism. These findings are the more noteworthy by reason of their contrast to the usual deep pigmentation of the Indian eye. Some of our specimens throw light on this phenomenon, for we observe in them two changes: (1) The pigmentary layer of the retina is irregularly thinned and altered, and at some points its pigment can be seen migrating into the choroid; (2) the choroidal pigment itself is extensively altered in an irregular manner, being heaped up in some areas and thinned in others. It is necessary to remember that, inasmuch as our specimens are wholly obtained from the coucher’s failures, whilst the interesting appearance we are discussing is best seen in his successes, we cannot expect very definite results from our pathological material, since the changes we desire to study are there overlaid and obscured by those of pathological processes, such as hypertony, hypotony, and inflammation.