256 



SENSE OF SIGHT. 



below it, (keeping the eye directed straight forward,) on moving the 

 candle slowly from right to left, (or if the candle be held on the right 

 side of the eye, it may be moved up and down,) a spectrum appears, 

 after a short time, in which the bloodvessels of the retina, with their 

 various ramifications, are distinctly seen projected, as it were, on a 

 plane without the eye, and greatly magnified. They seem to proceed 

 from the optic nerve, and to consist of two upper and two lower 

 branches, which ramify towards the field of vision, where a dark spot 

 is seen, corresponding to the foramen centrale. The origin of the 

 vessels is a dark oval spot, with an areola. This phenomenon must be 

 accounted for by the parts of the retina, covered by the bloodvessels, 

 not being equally fatigued with those that are exposed. 



It is by no means uncommon for appearances of cobwebs, small tubes 

 with lateral pores, &c., to present themselves before the eyes, without 

 changing their position when the eyes are fixed upon an object. These 

 appearances are not owing to any modification in the humours, but are 

 apparently dependent upon the physical condition of the retina. Some 

 years ago, a tube of the kind mentioned, but apparently terminating 

 in an open mouth, was the occasion of some uneasiness to the author. 

 This is now no longer seen, but numerous opacities, somewhat resembling 

 plexuses of vessels or nerves, are still apparent. All these appearances 

 are usually called collectively "muscse volitantes" 



They have been described by Mr. T. W. Jones 1 under three forms : 

 first, as a convoluted string of beads, or a 

 convoluted transparent tube, containing in 

 its interior a row of beads smaller than its 

 diameter, except here and there where one 

 larger than the rest is seen occupying its 

 whole diameter the end of the string or 

 tube sometimes presenting a dark knobbed 

 extremity, as if formed by an aggregation 

 of the beads composing the string, or con- 

 tained within the tube (Fig. 116, #); se- 

 condly, insulated beads, some of which 

 and these are more frequent, have a 

 well-defined outline 5; others, and these 

 are rarer, have a distinct outline <?; and 

 thirdly, a parcel of flexuous round watery- 

 looking or spun-glass-like filaments with 

 dark contours, often divided inferiorly into 

 truncated branches, d. 



The muscae, which change their position, 



Muse Voiitantes. would appear to be seated in the humours 



of the eye ; and it has been supposed in 



the vitreous more especially; whence the term ento-hyaloid muscde given 

 to them. 



It has been remarked, that the rays, proceeding from the upper part 

 of an object, impinge upon the lower portion of the retina; and those 



Fig. 116. 



1 The Principles and Practice of Ophthalmic Medicine and Surgery, Amer. edit., p. 323, 

 Philad., 1847. 



