and Locality of Muscle volitantes. 9 



system, and hence they cannot be regarded as of the same 

 character as the Muscce described in this paper, unless we 

 suppose that Musae, invisible under ordinary circumstances, 

 become visible in consequence of an increased sensibility of 

 the retina. 



This supposition, however, is by no means probable, be- 

 cause the Muscce 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 to direct experiment. With this 

 view, I examined the Muscce in the morning before the sensi- 

 bility of die retina had been diminished by exposure to day- 

 light, and found riiat they were neither increased in number 

 or intensity. I varied this experiment by diminishing the 

 sensibility of the retina. This was done by holding 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 refrangiljle ones*. In this case, too, the 

 Mufcce were as numerous and distinct as before, and we may 

 therefore consider it as certain, that the Muscce 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. I am disposed to think that they 

 consisted of the ordinary Muscce seen simultaneously with 

 others produced by the pressure of the blood-vessels on the 

 retina, and (hat it was the latter only which underwent the 

 variations which he describes. 



It is not easy to form any rational conjecture respecting the 

 cause and purpose of the numerous filaments by which the 

 Musccs are produced ; ibr as they exist in all eyes, whether 

 young or old, they are neither the result of disease, nor do 

 they indicate its approach. Were they fixed or regularly 

 distributed, we might regard them as transparent vessels which 

 supply the vitreous humour; but existing, as they do, in de- 

 tached and floating portions, they resemble more the remains 

 of those vessels, or of others whose functions are no longer 

 necessary. But though these filaments have no morbid cha- 

 racter, they may nevertheless obstruct and even destroy vision. 

 They certainly interfere with nice microscopical observations, 

 and in observing the minute and ahnost im})erceptible lines in 

 the solar spectrum, 1 have found them to be occasionally in- 

 jurious. It is (jiiite possible that some of the cells behind the 

 lens or even near the retina and around the optic axis might 

 be filled up with accumulated Mtisci:e,au(\ produce aconsider- 

 • Lond. and Edin, Phil. Mag., 1832, vol. i. p. i;2j vol.ii.p. 168. 



