626 A N A T 
The blood's velocity in the arteries is diminifhed during 
the heart’s fydole, but increased during its diadole ; at a 
medium its motion is fomewhat lei's than one foot in a fe- 
cond of time. The conflant plenitude of the arteries ren¬ 
ders it impoffible for us to perceive any fucceflion in the 
pulfes of different arteries ; whence all the arteries of the 
body feem to beat at one and tlj£ fame indant, whilft the 
heart (trikes againd the bread : and yet there is certainly 
a fucceflion in the fydole of the arteries, by which the 
aorta feems to contract fuccellively, as it is filled with 
blood expelled from the heart; fo that the part of the 
artery next the heart is firfi conftringed, and thence the 
arterial contracting force gradually proceeds to the extre¬ 
mities. We have an inftance of this in the intedines ; and 
very evidently in infects, who have a long fiftulous and 
knotted heart, manifeltly contracting in a fucceflion from 
the beginning to tire end ; but in the human arteries tli* 
fuccefiions are fo quick as to be imperceptible. 
The pulfe is continued to, and ends in, the expillary 
and cylindrical arteries, or the originations of the veins. 
We have already mentioned the velocity with which the 
blood comes from tire heart; but that velocity continually 
decreafes. The tranf’verfe feCtions of all the arteries at a 
difiance from the heart, are in one Cum greater than the 
fection of the aorta ; and the aggregate area of their fee - 
tions increafes, but in an uncertain proportion, as the dif- 
tance from the heart increafes : the velocity will confe- 
quently decreafe as the didance increafes, for it mud al¬ 
ways be inverfely proportional to the area of the tube 
through which the fluid runs. A conliderable allowance 
mud alfo be made for the great vifcidity or tenacity of 
the blood, which entirely coagulates by red; its circula¬ 
tory motion alone overcomes the mutual attraction of its 
parts, and prevents it from adhering to the (ides of the 
veflels in a coagulated date, as we fee in aneurifms and 
wounds of the arteries, and after death. The oppolition 
w hich the blood meets with in the branches, lelfens its 
velocity in the trunk: and the oppolition of torrents of 
blood to one another in the anadomofes of veflels alfo de- 
droys fome parts of its motion. We may eafily perceive 
the amount of this retardation will be very confiderable, 
although it be difficult to edimate it judly. In the larger 
trunks the blood of a living animal flows with the rapi¬ 
dity of a torrent: but, in the lead branches, it creeps 
along very (lowly, and begins to coagulate. It is alfo well 
known to furgeons, that a fmall branch of an artery near 
the heart bleeds more dangeroufly than a much larger one 
at a greater didance. The weight of the incumbent at- 
mofphere, of the mufcles and flefliy parts lying above the 
artery, and the contractile power of the veflel itfelf, alfo 
make a refidance to the heart; but they do not lelfen the 
velocity of the blood, for they add as much in the diadole 
as they diminiffi in the fydoie. The pulfe therefore en- 
fues, becaufe the anterior wave or column of blood moves 
on dower, while the lubfequent or poderior wave comes 
fader; fo that the preceding is an obdacle to -the confe- 
quent blood. But lince the force of the heart weakens as 
the blood goes on, and the contraftile power of the arte¬ 
ries increales, the excefs of the celerity of the conlequent 
wave pufhed on by the heart, above the celerity of the 
antecedent wave pufhed on by the contractile power of the 
artery, will grow continually lefs and lefs; and, when the 
blood arrives to a certain didance, the celerities of both 
waves become equal, and the pulfation ceafes. This plaice 
of equality in motion cannot be in the larger and more 
confpicuous arterial branches: for in them the wave lad 
coming from the heart moves quicker than what went be¬ 
fore. The inflammatory pulfation of the fmall arteries of 
the eye (hews that they have a pulfe. We may however 
fafely conclude, that in the lead red arteries the pplfe at 
length begins to vaniffi. This is evident from the equable 
motion of°the blood, as feen by a microfcope, through the 
arteries of a frog. In the larger veflels, however, fuch as 
may be about the fixth part of a line in diameter, the 
pulle becomes imperceptible. In the lead veins there is 
O M Y. 
no fenfible pulfation or accelerated motion of the blond, 
w'hild the heart contracts, demondrable either by the mi- 
crofcope or any other experiment. 
That the blood prelfes againft the fides of the veins, ap¬ 
pears from the furrows made on the bones- over which 
they-pafs, and the fwelling of the veins on being tied. 
Why then do not the veins beat ? The reafon feems to 
be, that the blood is more retarded immediately on its 
leaving the heart, than it is in the fmalled veflels. Hence, 
the difference of the velocities of the confequent and ante¬ 
cedent waves is greated at the heart, and grow s gradually 
lefs, till it at lad totally vanilhes. The pulfe is therefore 
the meafure of the powers w hich the heart fpends on the 
blood; becaufe it is the immediate and full effect of thofe- 
powers. Hence, catcris paribus , the pulfe is flow in the 
mod healthy people, where there is no himulus, nor any 
unnatural refidance; and where the heart is at liberty to 
propel the blood with eafe. We mud except thofe cafes 
where there is fome obdacle which px'events the blood from 
entering tire aorta. For this reafon the pulfe in adhmatic 
people is flow. A debility or infenfibility of the heart, 
when the ufual dimulus is not capable of exciting it to 
contraftion, alfo occafions a dow pulfe. A firong full pulfe 
is caufed by the arteries being full, and the heart at the 
fame time vigorous and powerful; a fmall pulfe by the 
emptinefs of the arteries, and a fmaller wave of blood fent 
from the heart. A hard pulfe denotes fome obdacle or 
dimulus : or elfe that the heart’s force, the thicknefs of 
blood, or the rigidity of the artery, are increafed. A quick 
pulfe denotes fome dimulus, obdacle, or greater fenfibi- 
lity or irritability of the heart. The pulfe is bed felt where 
the artery lies expofed bare to the touch, upon fome re¬ 
dding bone ; but obdruftions fometimes render the pullet 
perceptible where it is never fo naturally. 
The pulfe is (lower in animals as they are larger or more 
bulky; becaufe their heart, in proportion to the red of the 
body, is lefs than that of fmaller animals ; it is alfo lefs- 
irritable, and is obliged to propel the blood to a greater 
didance ; whence, in large animals, the proportion be¬ 
tween the relidunces to be overcome and the force of the 
heart is lefs than the fmall ones. Hence, fmall animals are 
more voracious than large ones; as the whale and ele¬ 
phant. The pulfe of a healthful perfon, in the morning, 
beats at lead 65 in a minute; but, after the fatigue of the 
day, it will beat So ; and again, by the night’s red or deep, 
it will become gradually lefs frequent, till in the morning 
you will find it returned to its primitive number of 65. For 
the motions of the tnufcles, and actions of the external 
and internal fenfes, the warmth of the atmofphere, and the 
atflion of the aliments, urge the venal blood on to the 
heart, whence a more than ordinary dimulus and a greater 
number of contractions. Hence alfo thofe paroxyfms, or 
fits of increafe, obfervable in all fevers towards the even¬ 
ing. Sleep retards the motion not only of the blood, but 
of all the other humours and actions in the body whatever. 
A frequent and a quick pulfe are often confounded ; 
but they are in reality very different. The pulfe is quicker 
in children, and becomes afterwards dower in perfons as 
they grow older. The falient point beats 13.4. in a minute : 
the pulfe of new-born infants, 120; and of old people, 60. 
A feverifli pulfe is ufually between 96 and 120, to which 
number indeed it is often increafed by laborious exercifes 
alone; if it is increafed to 130 or 140, (which lad num¬ 
ber we have never known it exceed,) the patient feldom 
recovers. The pulfe beats flower in winter, and quicker 
in dimmer, by about ten drokes per minute; and, under 
the torrid zone it often increafes to 120. The diderent 
paflions of the mind varioudy accelerate, retard, and dil 
turb, the pulfe. Whatever obdruCts the circulation,is alfo 
found to accelerate the pulfe ; not from the laws of hydro- 
datics, or on account of the canal being made narrower, 
nor from the a&ion of the foul; but by the drenuous and 
more frequently repeated contractions of the heart, in order 
to free itfelf from an irritating dimulus: -tints an irritation 
from acrid blood is tjie caufe of the frequent pulfe in fevers. 
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