PHYSIOLOGY. 
Iisis been called the nltimnm nioriens. Tiie 
blood, which returns by the venze cavae after 
the last expiration, no longer finds the usual 
passage through the lungs, which are con- 
tracted, but it is still urged on from behind 
by that which the aorta- has recently pro- 
pelled. Hence it is forced into the right 
auricle, and excites contraction in that part, 
by the stimulus of its presence, some time 
after the left side has been ,at rest. This 
congestion on the right side of the heart in 
the last agony explains the empty state of 
the arteries, pai'ticularly the larger ones, 
after death. 
It is hardly possible to determine the 
velocity of the blood’s motion in the healthy 
state ; for individuals differ from each other 
in this respect; and considerable variety 
probably takes place in different parts of 
the body. It is generally supposed, that 
the blood flows in a more gentle stream 
through the small arteries than in the arte- 
rial trunks; and that the velocity of its cur- 
rent is somewhat less in the veins than in 
the arteries of the body. These differences 
have, however, been exaggerated by former 
physiologists. The mean velocity of the 
blood in the aorta is calculated at eight 
inches for each pulsation, which gives about 
fifty feet in a miuute. If we reflect, that 
the systole of the ventricle, which gives the 
whole impulse to the blood, occupies only 
one-third of the whole pulse, the velocity of 
the blood’s motion must be trebled in that 
division of the time. It is said that this 
velocity, which we have assigned to the 
blood’s motion at its departure from the 
heart, becomes speedily diminished in its 
further progress; and the diminution has 
been deduced from various causes. The 
first and most powerful of these is the con- 
stantly increasing area of the branches, 
when compared with the trunk of an artery. 
(See Anatomy.) It is a well-known law 
in hydraulics, that the velocity of a fluid 
passing through an inverted cone constantly 
decreases, and that the diminution of velo- 
city is in the ratio of the increase of area. 
The mathematical physiologists have also 
noticed the effects of friction; deducing 
these from a comparison with the course of 
fluids in dead tubes. Other causes have 
been derived from the same source ; hence 
the ser{)entine course of some arteries, the 
unfavourable angles by which they some- 
times arise, and their comniunications with 
each other, are enumerated among the cir- 
cun)stances which retard the pourse of the 
arterial blood. But it must be remember- 
ed, that in viewing these retarding causes 
we are considering their action on the blood, 
as if this fluid were contained in inanimate 
tubes, and influenced merely by the con- 
traction of the heart, without taking into 
the account any accessory impulse which 
may be, and probably is, derived from the 
arteries. This retardation has been vari- 
ously estimated by different caleulators, 
who have all made it very considerable. 
Hales supposes the blood to flow through 
the capillary arteries of a frog, at the rate 
of two-thirds of an inch in a minute, which 
will be about 650 times slower than in the 
human aorta. Robinson and Whytt have 
gone still further : the former stating, that 
the velocity of the blood’s motion in the 
aorta is to that in, the smallest vessels as 
1100 to 1. We mention these calculations, 
to shew what absurdities- have been com- 
mitted by men of the greatest abilities, when 
they have applied the laws which regrdate 
the properties of dead matter to the living 
functions of the animal machine. Haller’s 
observations on the circulation in living 
animals (Elem. Phys. lib. vi. sect. 1, J. 30) 
entirely overthrow these calculations. He 
found by his microscopical experiments, 
that the blood flowed generally as rapidly 
through the small as through the larger ves- 
sels. He states also, that in living animals 
it is poured out as far from a small as from 
a large artery. Tlie numerous and diversi- 
fied experiments of Spallanzani afford addi- 
tional evidence of the same truth. 
We have stated, that the blood is thrown' 
into the arteries by separate contractions 
of the heart ; yet these vessels are constantly 
full, as may be proved, by opening them 
during the heart’s diastole. For the blood 
flows on in such a way, that the subsequent 
quantity discharged from the right ven- 
tricle, overtakes that which is before, and 
thus causes the pulsatio)i of the arteries. 
The excess of velocity in the blood coming 
from the heart, over that contained in the 
arteries, becomes constantly less ; and at 
a certain point ceases altogether. Here 
the pulse ceases also: hence in microsco- 
pical observations on the course of the 
blood in small vessels, its stream appears 
to be uniform ; and it is commonly stated, 
that the pulsation ceases in vessels of about 
one sixth of a line in diameter. 
'The motion of the blood in the minute 
veins, seem to be equal to its velocity in 
the small arteries ; this increases in the 
larger trunks ; and there is a constant ac- 
celeration in the blood’s course until it 
