25$ M V S 
the middle, and terminated on each fide by parallel black 
threads. On fixing the eye upon an objedl, they appear 
to defcend gradually. 
The fpots that appear fixed in refpedt to the axis of the 
eye, mult, for that reafon, be caufed by fome diforder in a 
correfponding 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 obfcurity or faintnefs 
of light in every place of the retina, and not a total defedt 
of it in any particular place. Hence M. de la Hire attri¬ 
butes the caufe of thefe permanent fpots to fmall 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 tranfinitted through 
a piece of bad glafs, and fall upon white paper, the Iha- 
dows of the little lands, veins, and irregularities in it, ap¬ 
pear not unlike thofe fpots. He therefore imagines the 
aqueous humour is fometimes troubled with fome little 
mothery or ropy fubltance ; fome parts of which, by the 
figures of their little furfaces, or by refradtive powers 
different from the humour itfelf, may caff their diftindt 
images upon the retina. He fuppofed them in the aqueous 
humour rather than the vitreous 5 becaufe of its greater 
fluidity for a freedom of defcent, and becaufe they will 
tljen appear to defcend, as being fituated before the pupil, 
or at leaft before the place of the interfedlion of the pencil. 
Pitcairn denies the caufe of the mufcse volitantes to lie 
in the aqueous humour ; and fays it is owing to, and a fign 
of, an internal inflammation of the veffels of the retina. 
Heifter thinks them owing to fome obftrudtion of thofe 
veffels 5 and both thefe authors, and many others, think 
them forerunners of a gutta ferena, or a cataradl; and 
propofe a cure by mercurials and decodtions of guaiacum. 
See Pitcairn’s Elem. Medic, and Heifter’s Compend. Medic. 
Pradtic. Plempius has given a defcription of the mufcse 
volitantes, in his Ophthalmographia. 
Dr. Porterfield, in the Medical Eflays of Edinburgh, ob- 
ferves, that thefe Ipots may proceed from fome little ex- 
travafations, various fwellings, or other defedls in the re¬ 
tina, which, by intercepting the rays, will occafion a de- 
J'edt in the pidture ; and by confequence a fimilar and cor¬ 
refponding defect or fpot in the object. He obferves alfo, 
that thefe Ipots commonly vanilli, or at leaft become Iefs 
fenfible, when the objedl: is brought nearer the eye, and 
within the limits of diftindt viiion. For the rays which 
are now exactly united upon the retina, by being more 
crowded, have their force augmented 5 by which means a 
fenfible impreffion is made upon the retina through thefe 
extravafations. Hence thefe fpots are molt fenfible to 
thofe who have a fmall pupil, and efpecially to thofe who 
are Ihort or long fighted. Hence alio, in the prelbytical 
eye, the fpots, which were formerly.very fenfible, become 
faint when the object is viewed through a convex glafs ; 
for by means of this glafs more rays enter the eye, which, 
being united exadtly at its bottom, muft ftrike the retina 
ftrongly enough to make a fenfible impreffion through thefe 
extravafations, which will render the fpots obfcure. The 
learned dodtor farther obferves, that what has been faid 
with refpedt to thefe fpots, when occafioned by extravafa¬ 
tions, or defedls in the retina, will alfo hold when they 
are occafioned by a callofity, or any degree of paralyfis or 
infeniibility in fome parts of the retina, by which tlieim- 
pulfe received from the rays is made lei's fenfible. 
Many medical authors have looked on the mufcas voli¬ 
tantes as forerunners of a cataradl, or gutta ferena. And 
we fuppofe it cannot be denied, that many who have fallen 
into thofe terrible dilorders have had thofe appearances 
before their eyes; and hence they might have been taken 
for prognoftics. But, as it is no lefs certain, that many 
thoufands have the fame appearances, and yet preferve their 
light very well to the laft, there feems to be no great reafon.- 
to be alarmed about them 5 and far lefs fiiould people be 
running to quacks, and injuring their ftomachs and con- 
M U S 
ftitutions by ill-judged phyfic on fuch occafions. St. 
Yves denies thefe appearances to be at all dangerous ; and 
he thinks them owing to the reparation of fome parts of 
the retina from the choroid. However, as the dilorder is 
troublefome and alarming to the patient, he advifes pur¬ 
gatives from time to time, and millepedes. We can fay, 
from experience, that neither mercurials, nor fea-water, 
nor otherpurgatives, nor millepedes, nor again Tunbridge 
or other chalybeate waters, have had any effedt towards 
removing or diminilhing thefe diforders. A greater dofe 
of wine than ufual has been known to increafe it for a 
little time; and Plempius has made the like obfervation. 
All authors agree, that thefe appearances change their 
figure, which is true in fome meafure ; but, though they 
feem, on a fudden looking to the Iky, or other bright olv 
jedl, to vary in their figures; yet, when they begin to de- 
fcend, they return in one conftant figure before we lofe 
fight of them. On the whole, it feems this phenomenon 
is not yet fully accounted for. The vafcular form of moft 
of thefe appearances, and their refemblance to fmall veins 
or arteries, make it difficult to conceive them floating in 
the 1 humours of the eye ; and, on the other hand, their 
feeming contortions, and change of figure, on fuddenly 
looking up, or thaking the head, make it no lefs difficult 
to conceive them fixed and obftrudted capillaries, as many 
authors fuppofe. 
MUSCA'RI, f. in botany. See Hyacinthus. 
MUSCA'RIUS, in heathen mythology,one of the names 
of Jupiter. 
MUSCATY, a town of Hindooftan, in Guzerat: fix- 
teen miles fouth-fouth-weft of Junagur. 
MUS'CAT, f. A delicious grape having the flavour of 
mulk ; a kind of l’weet pear. 
MUS'CAT, in geography. See Mascat. 
MUS'CHENBROECK (Peter de), a very diftinguiffied 
Dutch philofopherand mathematician, was born at Utrecht 
in the year 1699. He was fir ft profelfor of mathematics 
in his own univerfity; and afterwards invited to the chair 
of natural hiftory at Leyden, where he died full of repu¬ 
tation and honours in 1761. He was a member of feveral 
academies; particularly the Academy of Sciences at Paris. 
He was the author of feveral works in Latin, which are 
frequently referred to, and all of which difcover great pe¬ 
netration and exadtnefs in the fubjedts of which he treats. 
He was alfo confummate in the knowledge of law. The 
following are the titles of fome of his works : 1. Elements 
of Phyfico-Mathematico, 1726. 2. Elements of Phyilcs, 
1736. 3. Inftitutions of Phyfics, 1748. 4. Introdudlion 
to Natural Philofophy, 1762. Belides feveral tradls in 
the Memoirs of the Paris Academy, for the years 1734, 
35, 36, 53, 56, and 60. and feveral papers in the Phil. Tranf. 
vols. xxxiii. xxxvii. xxxviii. &c. 
MUS'CHLAU, a town of Moravia, in the circle of 
Brunn : twenty miles fouth of Brunn. 
MUS'CHUOF, a town of Arabia, in the province of 
Yemen : eight miles north-weft of Damar, 
MUS'CI, j. in botany, Mosses ; a moft diftindt and na¬ 
tural order of plants, of the Linnaean clafs cryptogamia, 
The word occurs in the belt Latin writers; but its origin 
is unknown. Ambrofinus deduces it, in his Phytologia, 
from ^oo-p/oj, “ a young calf or lamb,” as well as the 
“ young Ihoot or tendril of a plantwhich is not an im¬ 
proper etymology, and is adopted by Ray. The older 
botanifts have found ftill more difficulty in defining the 
nature of a mofs, than in the derivation of its name. 
Ray fays, " Moffes are belt diftinguiffied by their minute- 
nels ; their lefs fucculent, or even arid, fubltance; their 
power of reviving quickly, by immerfion in water, after 
they have been dried; their dry or abjedl fituations, ex- 
poled either to extreme cold or to fcorching heat, upon 
trees, timber, or ftones, where no other plants could e’xift 
or endure ; finally, their mode of producing fruit, which 
is unlike that of other vegetables.” Dillenius, who takes 
the idea of a mofs in its widell: fenfe, and who has illuf- 
trated the whole order, as he underftood it, in a moft care- 
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