AND LOCALITY OF MUSCLE VOLITANTES. 333 



While these results exhibit the true physical cause of all the optical pheno- 

 mena and limited movements of the filaments and Muscce, they lead also to some 

 important and useful conclusions of a more general nature. It had been conjec- 

 tured that the vitreous humour of animals was enclosed in separate bags or cells 

 connected with the hyaloid membrane by which the vitreous mass is enveloped. 

 The preceding experiments not only appear to demonstrate that this is the struc- 

 ture of the vitreous humour in man, but to shew that there are at least four or 

 five cells between the retina and the posterior surface of the crystalline lens. The 

 limited motion of the Muscce indicates that the cell in which they float is 

 of very limited extent. When the vertical diameter of the eyeball, in its 

 natural position, is placed, by the inclination of the head, 30 to the right hand 

 of a vertical line, and the optic axis of the eye directed 20 below a horizontal 

 line, the Musca is seen along the optic axis, and consequently in the most perfect 

 manner. One point of its cell must therefore touch the optical axis. 



I have endeavoured, with the assistance of my eminent colleague Dr REID, 

 to discover cells in the vitreous humour of quadrupeds and fishes by the aid of 

 the microscope and other means, but we have not succeeded : and unless some 

 chemical substance shall be found which acts differently upon the albuminous 

 fluid and the membranous septa, it is not likely that they will be otherwise ren- 

 dered visible.* 



Mr Ware, in a paper on the Muscce Volitanles of nervous persons,^ describes 

 some as " globules twisted together, and others as like the flue that is swept 

 from bedrooms," and he considers it " probable that they depend on a steady 

 pressure on one or more minute points of the retina which are situated near the 

 axis of vision .":): In the cases described by Mr Ware, the Muscce were liable to 

 great and sudden changes in intensity and number, particularly from causes 

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

 character as the Muscce described in this paper, unless we suppose that Muscce, 

 invisible under ordinary circumstances, become visible in consequence of an in- 

 creased sensibility of the retina. 



This supposition, however, is by no means probable, because 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 



* The vitreous humour, when slowly dried, either by itself, or along with parts of the septa in which 

 it may be contained, shoots into beautiful crystalline ramifications proceeding from the four angles of a 

 quadrilateral crystal. Thin six-sided plates frequently occur, but they seem to exercise no action upon 

 polarised light, probably on account of their thinness. The same effects were produced when the vitreous 

 humour from a fresh eye was well washed in distilled water. 



t Medico-Chirui-gical Trans., 1814, vol. v., p. 255. J Id. Id., p. 266. 



