1.28 The -BiO, OfK of N A’D UR Be or, 
himfelf to determine the manner in which 
the motion of the mufcles is performed;- 
neither was he bold enough to pronounce, for 
certain, that this motion proceeded from the 
influx or afflux of any new matter. But after 
I had, fome years ago, made him acquainted 
with the experiments I had made on this occa- 
fion, as already related, he made no difficulty 
of telling me plainly, that he was in no mea- 
fure afraid of abfolutely denying the acceflion 
of any new matter in the contraction of the 
mufcles ; fo that our opinions of this important 
operation perfectly coincide. 
' Even I myfelf, relying on the propriety and 
certainty of the experiments I have propofed, 
can now, without any difficulty, maintain, 
that a mufcle, at the time of its contraction, 
_ undergoes no inflation or tumefaction, from the 
afflux or effervefcence of the fuppofed animal 
fpirits; but that, on the contrary, it in this 
ftate becomes fmaller, or collapfes ; or, to ex- 
prefs my meaning more clearly, it takes up 
lefs room than it did before. 
Nothing can be more evident than this al- 
teration, when a heart filled with air in- 
ftead of blood, or one quite emptied, 1s made 
ufe of ; inthe firft of thefe experiments, there 
occur many other things worth our notice, 
which may all have place and force in the 
contraction of the mufcles : firft the enclofed 
air in the heart is condenfed, and forced toge- 
ther ; fecondly, the circumambient air is di- 
lated ; thirdly, the fibres of the heart are moft 
violently compreffed and {trained in this action, 
and the little cavities within, or between them, 
are comprefied ; fo that whatever has lain in 
thefe cavities muft be difcharged. And all 
thefe things appear chiefly at the time the 
heart relaxes, as it were for a moment in its 
contraction : fourthly, the internal air is af- 
terwards again rarified in the heart: fifthly, 
the external air is on the contrary condenfed, 
and driven from its place: and fixthly, the 
fibres of the heart are again extended or di- 
lated. 
If any one object, that the air in this cafe is 
out of the courfe of Nature in its place, I can 
readily anfwer, by affuring him, that I have 
found air in the hearts of human fubjects, 
opened immediately after their death. But as 
_ this is not a common cafe, I am content, that 
blood may be fubftituted to the air which I 
fuppofed to be found in the heart: the blood 
contained in the cavity of the heart, is on that 
cavity’ s contracting, fhaken, condenfed, and ex- 
pelled; the fame thing alfo happens to the 
blood which then flows through the coronary 
veins of the heart, and fhaken, or violently 
driven out of them; upoa which account alfo, 
the fubftance of the heart grows confiderably 
paler at this particular time. Now, while 
the heart is thus contraéted in its fubftance, 
the circumambient air is likewile rarified ; and 
laftly, the fibres which ferve to. move the heart 
violently, become powerfully contracted in the 
fame manner with the inflated heart, as already 
mentioned. But the laft appearances I took 
notice of on the fame occafion, and which 
were the reverfe of the firft, deferve to be here 
likewife confidered, as they act in the enfuing 
dilatation of the heart, when in its natural fitua- 
tion, and when it is moved in its natural 
manner. 
From all thefe particulars it evidently ap- 
pears, that there occur in the contraction of 
the mufcles, a great many more things to ex- 
ercife our reafoning and induftry, than authors 
have hitherto confidered. And above all, we 
ought moft carefully to obferve, with what 
ftrength the moving fibres of the mutcles draw 
themfelves up while the fibres are contracted. 
This power is fo great, that I have feen them 
in fome animals become three times fmaller 
than in their natural ftate of conftant con- 
traction. And on this account, all their con- 
tents, being all the blood and juices which had 
flowed into the blood-veffels running through 
them, were moft violently forced out by this 
powerful motion. Hence alfo it happens, that 
the mufcle of an animal, whofe blood is red, 
is much paler in a contracted, than in an ex- 
panded ftate. This D. Steno likewife has 
obferved. 
By this obfervation we are enabled alfo to 
account for the confiderable heat caufed in the 
body by the determinate and reiterated motion 
of the mufcles. For as the mufcles propel 
the blood from themfelves by their violent 
contractions, it is impoflible the motion and 
circulation of the mafs of blood fhould not 
be thereby confiderably increafed ; a circum- 
ftance which furgeons, who are informed of it 
merely by experience, have contrived in blood- 
letting, to take proper advantage of it; for at 
this time they give the patient a cafe of inftru- 
ments, or fomething of that kind, to employ 
his hands, that the mufcles being thus put in 
motion, the blood may more freely iflue from 
the veins: the imagination alone is often power- 
ful enough to have the fame effect, as our muf- 
cles are at that time varioufly influenced ac- 
cording to the lively or gloomy nature of the 
thoughts which then poffefs us, and propor- 
tionably contract and fhut, or expand our 
hearts. 
I faw myfelf, in the hofpital of Leyden, a 
boy from whofe feet the {kin and flefh had 
lately fallen off in a gangrene, who by the 
bare motion of his mufcles, without retaining 
his breath, could contrive to difcharge at will 
a confiderable quantity of blood from the 
wounded part. And I have likewife obferved 
the fame in the motion of many animals, 
whofe blood, tho’ they wanted lungs, flowed 
much more freely from them in motion, than 
when at reft. 
This cafe even extends fo far, as to account 
for laffitude or wearinefs itfelf, which is occa-+ 
fioned by the mufcles being too much diftend- 
ed by blood, and confequently rendered lefs fit 
for contraction, as. I firft obferved, in fhaping 
with my breath a piece of glafs melted with a 
lamp heat ; for the mufcles called Buccenatores 
in my face, came at laft to be fo violently dif- 
tended 
