Page:Cyclopaedia, Chambers - Supplement, Volume 2.djvu/97

 M U S

M U S

ous clafs ; but this feems very idle, as they are all trees, and have no refemblance with the Mufa, except in their having naked items. The leaves of the Mufa at firft have no re- femblance to thofe of the palms, but afterwards as they dry and wither they fplit on each fide into a vail number of fi- bres which have fome fort of refemblance with the palm- leaves.

The Indians diftinguifh no lefs than twenty-five fpecies of Mufa which they know from one another principally by the fruit. ' Philof. Tranf. N°. 410. MUSADA, a name given by fome chemical writers to fal-ar-

moniac. See the article Sal. MUSCA, the /Vy, in the hiftory of infefts. See the article Fly. Musca, in antiquity, a designation given to parafites, buffoons, &c. who intruded themfelves into the company of thofe that defpifed them, or found means to be admitted to entertain- ments where they were not welcome. Hofm. Lex. univ, in voc- Muscje is alfo ufed for officers of the popifh inquifition, who feign themfelves of the fed of the prifoners, in order to draw a confeffion of their opinion from them, and thereby betray them j there being perfons placed to overhear their conven- tion. Hofm. Lex. in voc. MusciE Volitantes^ dark irregular veins and fpots, feeming to fly before the eyes of many people, efpccially on looking at bright objects, fuch as white paper, the iky-light, the candle, iSc, Thefe have often the appearance of flies, Spiders, cob - webs, ibmetim.es that of fmoke, duff, &c. The refem- blance of many of thefe fpota to flies have given them the name of Mttfca Volitantes, q. d. flics flying about. This phenomenon is not eafy to account for. Some will have its caufe to lie in the aqueous humor ; others in the re- tina. Monf. de la Hire's account of them is this : They are of two forts, fome permanent, which, in fixing the eye upon a point of an object, appear always fixed in the fame fitua- tion to that point - 3 others feem to fly about, and to change their fituation, though the eye be fixed. The fhapes of both forts are changeable. The firft fort are commonly like a dark Spot upon a white ground ; the fecond fort appear like the knots of a deal board ; fome parts of them being very clear and furrounded with dark threads ; they are alio attended with long fillets of irregular fhapes, which are bright in the jniddle, and terminated on each fide by parallel black threads. On fixing the eye upon an objea, they appear to defcend gradually ; efpecially after making the head Suddenly. The fpots that appear fixed in relpecl to the axis of the eye, muft, for that reafon, be caufed by fome diforder in a corre- sponding part of the retina, or in fome part of the vitreous humour lying pretty clofe to the retina. For an opacity of the coats or humours in any part remoter from the retina, by intercepting fome part of the rays of every pencil, could only caufe an uniform obfeurity or faintnefs of light in every place of the retina, and not a total defect of it in any particular place. Hence M. de la Hire attributes the caufe of thefe permanent fpots to Small drops of extravafated blood upon the retina.

But he finds it more difficult to account for the moving fpots. When the rays of the fun are tranfmitted through a piece of bad glafs, and fall upon white paper, the fhadows of the little Sands, veins, and irregularities in it, appear not unlike thofe fpots. He therefore imagines the aqueous humour is fome- times troubled with fome little mothery, ropy fubftance ; fome parts of which, by the figures of their little Surfaces, or by re- fractive powers, different from the humor itfelf, may caft their diftinft images upon the retina. He fuppofes them in the aqueous humour, rather than in the vitreous ; becaufe of its greater fluidity for a freedom of defcent, and becaufe they will then appear to defcend, as being Situated before the pu- pil, or at leaft before the place of the interferon of the pencils. But if this heterogeneous mother be in the vitreous humor, it muft be lighter than this humor, fo that after a fudden make of the head, it may firit defcend a little, and then afcend gradually, to caufe the gradual apparent defcent above-men- tioned. La Hire Difert.fur Us differ, accident de la vue. Thefe fpots are obferved^ to change their figures, Sometimes m two or three hours, at other times not in two or three days j and to appear more numerous at one time than another. Id. ibid, and Dr. Smith Optics in the Remarks. Art. 27. Seq. '

We have an inflance in the Philofophical Tranfaaions, N°. 384.. of thoSe fixed fpots above-mentioned. A woman, who, in looking with the left eye only, at three fhort words in print, could fee the extremes, but not the middlemoft j and, in look- ing with the right eye only, at the middle between four fhort words could fee but three of them ; one of the two middle- molt bemg covered with a dark round Spot; but in looking with both eyes {he could fee them all.

Pitcairn denies the caufe of the Mufa volitantes to lie in the aqueous humour, and (ays it is owing to, and a Sign of, an internal inflammation of the veilels of the retina. Heiiter ™" ks . th . em °w»ng to Someobftruaion of thofe veffels; and ootn t&ele authors, and many others, think them forerunners ot a gutta Terena, or a cataracl j and propofe a cure by mer- cunals and decorous of guaicum. See Pitcairn's Elem. Me-

dic. & Heifler's Compend. Medic. Praaic. Plemptcs has given a defcription of the Mufca volitantes, in his Ophthal- mograph ia.

Dr. Porterfield, in the Medical Effays of Edinburgh, obferves, that thefe Spots may proceed from Some little extravafations, verrucoSe fwellings, or other defeas in the retina, which by in- tercepting the rays will occafion a defea in the piaure ; and by confequence a Similar and correfponding defeat or fpot in the objea. He obferves alio, that thefe fpots commonly vanifh, or at leafl: become lefs fenfible, when the objea is brought nearer the eye, and within the limits of diftina vilion. For, the rays which are now exaaiy united upon the retina, by being more crowded have their force augmented ; by which means a fenfible impreflion is made upon the retina through thefe extravaSations. Hence thefe fpots are moft fenfible to thofe who have a fmall pupil, and efpecially to thofe who are fhort, or long-Sighted. Hence alfo, in a prefbytical eye, the fpots which were formerly very fenfible, become faint, when the objea is viewed through a convex glafs ; for by means of this glafs more rays enter the eye, which being united exaaiy at its bottom, muft Strike the retina ftrongly enough to make a fenfible impreflion through thefe extravafations, which will render the fpots obfeure.

The learned dodtor farther obferves, that what has been Said with refpea to thefe fpots, when occaiioned by extravafations or the defeas in the retina, will alfo hold when they are oc- casioned by a callofity, or any degree of paralyfis or infenfibi- lity in fome parts of the retina by which the impuife received from the rays is made lefs fenfible.

But, befides thefe defeas in the retina, there is another caufe which may give occafion to thefe fpots, both in the myopia and vljus fcnilis ; and that is, certain fmall opaque marks in the cornea itfelf, or any where within the eye, which, by in- tercepting fome of the rays, mult occafion a defea in the pic- ture, from which defea a kind of dark Spot will be fcen in the objea. Thus, in the eye of a myops, if there be any opacity in the cornea, or within the eye which intercepts the rays Bbe t Ccd, and VLa, there will be a defea in the

piaure at e, d, and a ; from whence alfo the external ap- pearance will be deficient at the correfponding points E, D, and A ; where, by confequence, dark fpots will be fecn. For it is to be obferved, that there is not one point in the piaure which is formed by a plurality of rays which meet at the point, but every ray goes to a different point of the piaure, both in myopical and prefbytical eyes ; and therefore, when any of the rays are intercepted, that part of the picture to which fuch rays belong will not be illuminated, which muft occafion a corresponding defea in the appearance of the ob- jea. But in the perfea fight, where the rays which come from the Several points of the objea, are fo refra&ed as to meet again at fo many corresponding points in the retina* every point of the piaure is formed by a cone of rays, whofe bafc.is the pupil ; and therefore, though fome of thofe rays be intercepted, yet no part of the piaure will be darkened, and confequently no defea will be Seen in the objea from any fuch opacity in the cornea, or humors of the eye, unlefs this opacity be in the back part of the vitreous humor, where the pencil is narrow, and intercepts the whole pencil. This may be proved from the Camera Obfcura, by flicking fome Small patches on the glaSs. Thefe will not be perceived if the paper is placed at the due focal diftance ; but if the diftance be greater or fmaller, the fpots will appear.

The doaor alfo obferves, that thefe fpots are not all of the fame kind. He thinks that thofe which change not their place with refpea to the axis of vifion proceed from fome de- fea either in the retina or cornea, or in the vitreous and cry- ftalline humors. As for thofe which are in conftant motion, he Suppofes them to arife from fome corpufcles floating in the aqueous humor.

Befides thefe dark fpots, there is another kind often met with, more bright and luminous than the objea itfelf before which

they