404 
ON THE DAILY LABOURING FORCE OF THE 
HUMAN HEART 
INK 
T remains for me to explain the manner in which the 
two elements of the calculation of the daily labouring 
force of the heart (see p. 255) have been ascertained. These 
elements are, the capacity of the left ventricle of the heart, 
and the hydrostatical pressure of the blood inside the 
ventricle at each pulse. 
The average capacity of the left ventricle is ascertained 
by filling it with melted wax, at a pressure equivalent 
to that of oft. vertical, of blood ; and afterwards weigh- 
ing the solidified wax cast of the ventricle and com- 
paring its weight with that of a known volume of 
the same wax. In this manner, it has been found that the 
average capacity of the left ventricle does not differ much 
from 3 ounces. 
In the unavoidable absence of any direct experiment on 
the hydrostatical pressure of the blood in human arteries, 
we are obliged to have recourse to indirect methods of 
estimating its amount. The first attempt made by me 
was the following :—On the 22nd of March, 1863, I had 
an opportunity of witnessing the removal of a large fibro- 
cellular tumour from the left groin of a middle-aged, large 
sized man, in the operating theatre of the Meath Hospital. 
In the course of the operation, the external epigastric 
artery (which appeared enlarged to feed the tumour) was 
divided, and before it could be ligatured, strong jets of 
blood were thrown from it in various directions about the 
floor of the theatre. I noticed, as the poor fellow struggled 
on the operating table, that the jets of blood fell short, or 
enjoyed a longer range, according to the angle of elevation 
of the orifice of the bleeding artery, and that there was a 
certain maximum range on the floor of the theatre, which | 
was not exceeded. Having afterwards measured the 
vertical height of the bleeding artery, and the horizontal 
distance of the squirts of blood corresponding to the 
maximum range, I found them to be 3 ft. 6 in, and 8 ft. 
respectively. From these dafa,I readily calculated (by 
the parabolic theory of maximum range of projectiles ona 
descending inclined plane) the velocity of the blood issuing 
from the orifice of the artery, and found it to be 12905 ft., 
corresponding to an hydrostatical pressure of 2°586 ft. 
This result, although of great value, leaves us still in 
ignorance of the hydrostatical pressure of the blood in- 
side the arteries when they are intact; for, owing to the 
wonderful perfection of the mechanism of the heart, its 
force of contraction is exactly regulated by the resist- 
ance it is compelled to overcome, and as soon as a 
large artery is opened, the heart instinctively feels that 
the resistance is lessened, and spontaneously reduces its 
force of contraction, to correspond with the diminished 
resistance of the circulation. The beneficial effects of 
this remarkable property of the heart, in the case of 
wounded arteries, are evident, for its reduced force of 
contraction greatly diminishes the loss of blood. 
Dr. Hales, in the course of his Haemastatics, remarks 
that the blood did not spout much higher than 2 feet from 
the wounded artery of the horse, although the pressure 
inside the arteries, when the circulation is complete, 
exceeded 9 feet of blood. The difference in the force of 
the heart in the two cases arises from the resistance 
offered by the capillary circulation. 
We find ourselves, therefore, obliged to estimate the 
force of the heemastatical pressure in the human arteries, 
not by direct experiment, but by the following indirect 
reasoning. 
The experiments of Poiseuille on the discharge of 
liquids through capillary tubes, prove that the resistance 
offered by such tubes is directly proportional to the length 
of the tubes and inversely proportional to the squares of 
their cross sections. 
NATURE 
[ Fed. 17, 1870 
_ The quantity of liquid discharged by a capillary tube 
in a given time is inversely proportional to this resistance, 
and may be expressed by the following formula :— 
Oa yt 
Z 
In this expression Q denotes the quantity of liquid dis- 
charged in a given time, A is a constant, 2 denotes the 
charge or hydrostatical pressure, and @ and / are the 
diameter and length of the capillary tube. 
Now, there is reason to believe that in animals, similar 
in bulk, the arrangement and structure of the capillaries 
are such that the ratio of the squares of their cross sections 
to the total lengths of the capillaries is practically con- 
stant, as may be proved from the following comparison 
of the sheep and dog. The left ventricle of a sheep’s 
heart, according to Hales, contains 1°85 cubic inches, and 
its pulse beats 65 times in a minute;.the quantity of 
blood passing through its capillaries in a given time being 
obviously proportional to the product of these two quan- 
tities. The heemastatical pressure in the arteries of the 
sheep (Hales) is 6°46 feet of blood. 
If we bring to the left hand side of equation (1) the 
quantities depending on capillary resistance, we find 
GOW ares 65 
| The number thus found is to be regarded as the capillary 
coefficient of the sheep. The average of the capacities of 
the left ventricles of six dogs measured by Dr. Hales, 
was 0'954 cubic inches ; and the average hzemastatical 
pressure in the arteries of sixteen dogs, was 4°75 ft. of 
blood; while the pulse of the dog beats ninety-seven times 
in the minute, on an average. Hence we can obtain the 
capillary coefficient of the dog, 
a Q__0'954 X 97 
ge eS 
The sheep and dog differ from each other, as much as 
man and the horse do, in size of heart and rate of pulse ; 
they also differ in hamastatical pressure ; yet, notwith- 
standing these differences, the cafz//ary coefficient depend- 
96. 
_ ing on them all, comes out to be nearly the same in both 
animals. 
The capillary coefficient of the horse is double that of 
the sheep and dog, showing that the resistance to circula- 
tion in the horse is only half that of the smaller animals. 
| The left ventricle of the horse contains 10 cubic inches, 
the rate of pulse is 36 beats in a minute, and the average 
hemastatical pressure is 914 ft. of blood. Hence we 
find for the capillary coefficient in the horse 
a Q ~10 Xs ‘ 
AX ex if >= Cam. 2s 
I now assume that the capz/lary coefficient in man is the 
same as in the horse; or, in other words, that man bears 
to the horse, in regard to blood circulation, the same 
relation as the dog bears to the sheep. 
On this assumption, the hamastatical pressure in the 
human arteries may be thus found. The human heart 
has a capacity, in its left ventricle, when in action, of 3 
ounces, or 5'2 cubic inches, and beats 75 times in a 
minute. Solving equation (1) for 4, we find 
O 
h = = 
a 
AX i 
Substituting for O, the product of 
tricle and rate of pulse; and for 
its value in the horse, we obtain 
pew x 75 
the capacity of the ven- 
the capillary coefficient, 
—-39°3 
This is the hamastatical pressure used in the preceding 
paper on the force of the heart. 
= 9'923 ft. of blood. 
SAMUEL HAUGHTON 
