PHYSIOLOGY. 
arrives at the heart. This fluid is passing 
through tubes which constantly decrease 
in area; and it follows of necessity, that by 
diminishing the channel of a fluid, its course 
must be accelerated. Hence the trunks of 
the venae cavae return to the heart, within a 
given time, as much blood as the aorta 
carried out of this viscus. 
The motion of the blood rfong the veins 
must be derived from the impulse which it 
receives from the heart, and from the ac- 
tion (if there be any) of the arteries. Its 
circulation in these vessels is aided by the 
contraction of the muscles, which must 
urge on the contained fluid towards the 
heart ; since their valves prevent any retro- 
grade motion. 
We shall readily perceive, that no certain 
calculation can be formed of the powers of 
the heart, when we consider that neither 
the quantity of blood expelled at one pul- 
sation ; nor the distance through which it 
passes in a given time ; nor the velocity of 
its course can be defined with any certainty ; 
much less can we form any accurate esti- 
mate of the obstacles which occur to the 
blood’s motion; which must considerably 
affect such a calculation. We may how- 
ever approach in some degree to the truth, 
by collecting and comparing the results of 
probable conjecture. If we calculate the 
blood contained in the body at thirty pounds, 
the number of pulsations in one minute at 
seventy-five, and the quantity expelled from 
the left ventricle at each pulsation at two 
ounces and a half, the whole quantity will 
pass through the heart about twenty-two 
times in the course of an hour ; and it will 
perform the circulation once in less than 
three minutes. The velocity with which 
this blood is propelled by the systole of the 
left ventricle may be collected from the 
violence with which it is ejected from a 
wounded artery ; and the altitude to which 
it ascends. Blumenbach has seen it pro- 
jected more than five feet from the carotid 
of an adult during the first contractions of 
the heart. Our countryman Hales calcu- 
lated from his experiments, in which he 
measured the height of the blood’s ascent 
in a glass tube, inserted into a large artery, 
that it would be thrown seven feet and a 
half from the human carotid : he estimates 
the surface of the ventricle at fifteen square 
inches ; and thus finds that one thousand 
three hundred and fifty cubic inches, or 
about fifty-one pounds weight, press upon 
tlie left ventricle, and must be overcome 
by its systole. Many oth^er (jsdculationjs of 
the powers of the heart have been formed 
upon mathematical principles ; but different 
persons have been led to such opposite re- 
sults, that we are warranted from this cir- 
cumstance in disregarding them altogether. 
Borelli makes the powers of the heart equal 
to an hundred and eighty thousand pounds ; 
Keill to eight ounces. Senac observes, 
that if a weight of fifty pound be attached 
to the foot, with the knee of that side placed 
on the opposite one ; the weight will be 
elevated at each pulsation : this weight is 
placed at a considerable distance from the 
centre of motion; and, allowing for this 
circumstance, he estiipates the moving 
power at four hundred pound. 
This power of the heart, so wonderful 
both in extent and duration, must be re- 
ferred to the irritability of the organ, in 
which point of view it seems far to exceed 
all other muscular parts of the body. That 
the immediate cause of contraction in this 
viscus arises from the presence of blood in 
its cavities, is shewn by the celebrated 
experiment of Haller ; in which' the longer 
duration of action in the right or left cavi- 
ties, was varied by influencing the supply 
of blood. 
In the action of those muscles that de- 
pend on the will, , a supply of nerves, and 
a distribution of blood to the moving fibres, 
seem to be essential conditions. It has 
been disputed whether or not these cir- 
cumstances are necessary in the heart, and 
what share they may contribute to the 
heart’s action. We may observe in the 
first place, that the actions of the heart are 
completely involuntaiy ; that no exertion 
of the will can produce the smallest effect 
in accelerating, retarding, or otherwise af- 
fecting the actions of this part. Yet vari- 
ous arguments prove that the nerves exert 
an influence over this organ. Not to men- 
tion the peculiar arrangement of the car- 
diac nerves ; the sympathy betvveen the 
heart’s action, and nearly every other func- 
tion, even of the most different classes, suf- 
fices to demonstrate the connection. The 
vehement disturbance of the heart from 
the passions of the mind, must be familiar 
to every person from his own experience ; 
its action is also strongly influenced by 
various states and affections of the alimen- 
tary canal. 
The action of the heart is intimately con- 
nected with the changes which the blood 
undergoes in its passage through the lungs, 
for when respiration is obsti’ucted, the 
heart’s action ceases, and it may be re- 
