70 - Proceedings of Learned Societies. 
ones; and the principal part of the re- 
gs arises from these’small vessels. 
Fhe tiocd in the arteties is subjected to 
a certain pressure, by means of which it 
is forced into the veins, where the tension 
35 muctr less cdnaitervbhe: and this pres= 
sure, originating from the contractions 
of the Neat, and coitinued by the ten- 
sion of the arteries, iS almost entirely 
ia 
employed in overeoming the friciton’ af 
the vessels: for thie toreé required to 
overcome the inertia of the blaad isso 
trifing, that it may be neglected. Hence 
fie enquiry refers to the magnitude of 
the pressure, and to the degree of re- 
sistance which arises from thie friction 
of the internal surface of the blood-ves- 
sels, or from any other caases of re- 
tardation. Dr. Young refers here to 
the experiments of ” Hales, and “other 
philosophers ; and likewise to many of | 
his own, ap account of ‘which has been — 
lad hele the Royal Society; and. he 
observes, that there can be no doubt 
that the resistance of the interhat Sur=— 
tace of the arteries, to the motion of tlie 
blood, must be much greater than would 
Be‘found in the case of water; supposing 
it four times as great, arid it will give 
eivbty inches for the measure of a co- 
Jumn, of which the pressure is capable 
oF forcing the blond in its natural course, 
through the smaller arteries and veins. 
Dr. Young next exa amines the nature 
and velocity of the propagation of the 
pulse. The successive transmission of 
the pulsations of the heart through the 
leneth of the arteries is $0 analogous £0 
the motion of the waves on the surface - 
af the water, or to that ofa sound trans- 
mitted through the air, that the saine 
calculations will serve for ‘determining 
the principal affections of all these kinds 
of ‘motions; and if the water, which is 
- agitated by waves, is supposed to flow at 
the same time in a continued. stream, 
aud the air which conveys a sound to be 
earried forward also in the form of 
«ind, the sumilitude will be still stronger. 
The coats of the arteries. may perhaps be 
considered as peitectly elastic ; that is, 
as producing a force proportional to the 
degree in which ey are extended, 
beyond then natural di imensions ; but 
there may be bodies im nature, which 
differ materially from this general law, 
‘especially where the distension becomes” 
evnsiderable: thus. there may -be sub- 
stances which exhibit a force ‘of tension, 
propartional to the excess of the square, 
arthe cube, of their length, beyond a 
certain given quantity. 
_ only about one third part. of the i ‘5 
_ between two successive pulsations ; and 6." 
- [Aag. 1, 
Tr. Young again’ refers to the expe- 
riments of Hales, and concludes, that the 
velocity of the ptilse must.be nearly the 
same as that of an impulse, transmitted 
through an elastic fluid, uuder the pres- 
sure of a column ‘of the same height, as 
that which measures the actual arterial 
“pressure; that is, equal to that which is 
acquired. by a heavy body falling freely 
through half this hehe. In maa, this 
velécity becomes about fifteen and a 
half feet in a second; to which-the pro- 
gressive motion of the blood itself adds 
about eight inches; and with this velo- 
city, of at least sixteen feet in @ Second, 
it thay easily happen that the pulse may 
appear to arrive at the most distant 
parts of the body, without the interven- 
tioh Wd any pereeptible interval of time. — 
The velocity of the transmission of the 
pulse being known, ‘it is easy to Geter- 
mine the degree i in which the arteries are 
dilated, during its.passage through t them, ~ 
heise wetting of | he blood, i in the | 
, being eicht and a half inches in a 
ptr its greatest ‘velocity must he 
three times as much, since the contrac- 
tion of the heart. is supposed to occu: 
if the velocity of the pulse is sixteen feet 
in a second, that of the blood itself must 
be about ‘one-eighth. part as gregt, 
that the column of blood — 
occu 
eight inches, may occupy only s seven; ” 
hence the diameter must increase it the 
ratio of about fifteen to sixteen. 1 
‘tension will be also become. one-eighs th 
-quarter, and the force of the heart must ~ 
be capable of supporting a column Of oné- 
“handred and one inches; nor would this 
force by any means require ; a strong €x- 
értion of muscular power; for it “only 
implies a tension of - something. fegs than. 
three pounds for each inch of the cir- 
cumference, of the greatest section oh 
the heart; and supposing _ the meas 
thickness half an inch, an equal vamber 
6f the bres of some Orhar muscles of the 
body, would be capable of éxerting a * 
force of more ‘than two bund id pounds; — 
in * the ‘state of the oie spel . 4 
action. 
In enquiring into. the’ natare and ¢ a 
tent of the functions, which are to be " 
attributed ‘to the muscular fibres of the- 
coats of the arteries, Dr, Young stipposes 
it certaiy, that tley are much less con- 
_ cerned in the progressive motion of (fig, 
blood, ‘than is generally supposed ; for 
this; opinion he adduees a variety of 
strong arguments ; « and adds, “ By 
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