The BOOK 
and then fcratched the fkin until he made it 
red, imagining that it was probably occafioned 
by a Flea, or fome other infect. But when I 
ftopt, his hand and arm refted likewife, becaufe 
the natural contraction was then equal in all the 
mufcles, If the fame experiment be made on 
fleeping dogs or cats, it is likewife obferved, 
that a determinate motion is produced in the 
mufcles which move their fin; and therefore 
it is pleafant to fee them fuddenly draw it up, 
pricking up their hairs, and fometimes fkaking 
them in their fleep. By this familiar inftance, it 
is evident how our mufcles are in like manner 
voluntarily moved without any great attention of 
the will, by fomething of this kind, which is 
proper to determine the natural motion of the an- 
tagonift to the contrary fide. 
Now, in order tomake it certain from expe- 
riment, that the mufcle is not inflated in its con- 
traction, but on the contrary poffefles lefs {pace, 
we are to take a very lively and found Frog, and 
diffe&t it, uncover its heart, and carefully take 
away the pericardium from it. After this, we 
mutt choofe one or two veins or arteries, which 
are large enough to be conveniently opened, 
and to admit a thin glafs tube. By this tube, 
all the veins and arteries of the body, and con- 
{equently the heart, may be very eafily inflated ; 
fince, as I have before hinted, the lungs are in 
this animal no obftacle. 
When the heart is filled with air, it muft be 
dexteroufly, together with its auricle, tied with 
a fine thread, and cut away from the body: then 
let a glafs fiphon be ready, one end of which 
muft be a narrow and {mall tube. Let the heart 
thus inflated, and its auricle, be put into the flat 
end of this tube, and let all be immediately put 
into that glafs fiphon; the long tube of which 
muft, in the mean time, be ftopt with a very 
{rnall drop of pure water, or, that it may be the 
better diftinguifhed, water coloured with blood. 
Having duly obferved thefe directions, it will 
manifeftly appear, that at the time the heart, 
Tab. XLIX. Fig. vir. a, contracts itfelf within 
the little fiphon, 44, the drop of water adher- 
ing near the extremity of the tube, ¢, defcends 
in a very remarkable and furprifing manner to the 
other end of the tube, d, where it {prings from 
the fiphon; and, on the contrary, it will like- 
wife diftin€tly be feen, that the drop thus fallen 
down, d, will, on the heart’s dilating itfelf again, 
rife to its former fituation, ¢. 
This experiment furnifhes us with an evident 
proof, that when the mufcle of the heart con- 
tracts itfelf, not only all the fibres which ferve 
to move it, are preffed clofer to each other, but 
that the heart itfelf alfo, occupies a fmaller {pace 
in its fyftole, than it did before in its diaftole. 
This alfo is the reafon why the drop of water 
c moves downwards, d, as it cannot but follow 
the heart, when this contracts itfelf. But HM at 
the very inftant of the heart’s contraction, any 
inflation, tumefaction, or dilatation, had been 
produced by the animal fpirits on the infide of 
this organ, the drops, inftead of defcending to- 
wards the belly of the fiphon, d, would infal- 
feo 
of NeA OU RIES or, 
libly have rifen towards the extremity of the 
tube, e. 
As the former never happens ; and the latter, 
the very reverfe of it, is conftantly the cafe, | 
may fairly and plainly conclude, that the mufcle 
of the heart requires lefs room by a great deal in 
its contracted, than in its dilated ftate; and that 
hence the fuppofed {pirits, by which it hus been 
hitherto believed, that the heart, or its mufcle 
was puffed up at the time of its fyftole, has not 
the leaft fhare in producing that effect. 
Moreover, if we open a living Frog, and care-~ 
fully attend to the motion of its heart and the 
auricle, we fhall find, that in this experiment, 
every thing proceeds exactly in the fame man- 
ner as it didin the former. For, when the au- 
ricle contracts itfelf, it very fenfibly grows {maller, 
and more compact, ; but when the heart is again 
contracted, it undergoes the fame alteration; and 
this obfervation is fufficient to convince us, that 
there is no manner of difference between the two 
contraGtions of the heart, one of which takes 
place within the fiphon, and the other naturally, 
except that the heart, out of the fiphon, is in- 
flated with blood, and with air in the fiphon. 
As to the other experiment made in the 
fiphon, we muft here particularly obferve, what 
happens in the heart during its dilatation, and 
what change is feen afterwards during its con- 
traction. When the heart dilates itfelf, we plainly 
perceive that the auricle begins firft to contract, 
and while it does this, the air is forced from it 
into the heart; by which means it is confiderably 
expanded, and appears in the fiphon as if full of 
bubbles or bladders of air. It even becomes 
pale and tranfparent on this occafion, and ap- 
pears irregularly affected. ‘This is owing to its 
moving fibres, and flefhy columns, not being 
every where of the fame thicknefs, fo that fome 
of the parts of the heart lying between thefe co- 
lumns, are more diftended by the impelled air 
than others. Thus at length is effected, the 
afcent of the drop of water adhering to the glafs 
tube. 
When the moving fibres of the heart again con- 
tract themfelves, we obferve that the heart draws 
itfelf in, and becomes fmaller, and immediately 
after, we fee the air forced from it, in its turn 
into the heart; upon which this laft immediately 
becomes more red and opaque, and fhrinks up fo 
as to put on an unequal appearance: but as the 
heart at this time cannot return to the auricle, 
all the air it had thence received, its moving 
fibres approach towards each other with fo vio- 
lent an effort, that they likewife condenfe the 
air contained in them: and thus is the drop of 
water adhering to the tube of the glafs fiphon 
prefled downwards, on account of the heart be- 
ing then reduced to a lefs fize. 
This is likewife the cafe with the heart, which 
is naturally full of blood; for when this organ 
in its fyftole is diftended by the blood, it draws 
away the circumambient air ; but when it again 
contraéts itfelf, and difcharges its blood, it grows 
lefs, and yields to the air in proportion as it 
fhrinks up, a thing which ought to be well at- 
tended 
