On the vision of objects on and in the eye / by William Mackenzie.

  • William Mackenzie
Date:
1845
Catalogue details

Licence: Public Domain Mark

Credit: On the vision of objects on and in the eye / by William Mackenzie. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    denly than some authors have represented. Hence the question, whether, invisible under ordinary circumstances, they become vi- sible in consequence of an increased sensibility of the retina. “ This supposition,” observes Sir David Brewster, “ is by no means probable, because the muscae are not visible by any light of their own, and an increase of sensibility in the retina would affect equally the luminous field on which they are seen. But as this point is of some importance both in a physiological and a medical aspect, I have submitted it,” he goes on to say, “ to di- rect experiment. With this view, I examined the muscae in the morning before the sensibility of the retina had been diminished by exposure to day-light, and found that they were neither in- creased in number nor intensity. I varied this experiment by di- minishing the sensibility of the retina. This was done by hold- ing a bright gas flame close to the eye, and near the axis of vision, till the retina lost its sensibility to all the rays of the spectrum, except a few of the more refrangible ones. In this case, too, the muscae were as numerous and distinct as before, and we may therefore consider it as certain, that the muscae described by Mr Ware, in so far as they were of the same character as those in the healthy eye, are not affected by any variation in the sensibility of the retina.” To recur to one of the most certain exciting causes of pearly muscae, How do the ophthalmias, it may be asked, lead to this disease ? It is well known, that in every severe ophthalmia, what- ever be the texture primarily or principally affected, the whole blood-vessels of the eye become injected. In iritis and retinitis, this is attended with dimness of sight, often to a great degree ; in iritis, from effusion of opaque lymph into the pupil, and in re- tinitis, from a similar effusion on the surface of the retina or into the vitreous body. As the effusion in either case is removed, vi- sion clears, but of muscce volitantes the patient now almost in- variably complains. Tt has already (§ 23,) been stated, as a con- jecture, that the most common kind of muscae, the pearly, may perhaps be the result of the blood-vessels, which run forwards through the vitreous body towards the lens, becoming perceptible, by means of their diffracted shadows, and that the circumstance of their being seen much more by some than by others should be attributed to their obliteration being less complete in some sub- jects than in others. If this conjecture be true, may not the oc- currence of muscae after a severe ophthalmia be the consequence of the contracted vessels yielding to the push of the blood from the centra] artery of the retina, of which the vessels in question are branches, so as to readmit a certain quantity of blood, which either remaining there or becoming effused, gives rise to musca volitantes ?