363 
PHYSIOLOGY. 
you ftrike tile end of a long beam, the motion is quickly 
communicated to the oppofite extremity. Hence, if the 
arteries of a dead body are filled witli water, and an addi¬ 
tional quantity is forced in by means of a fyringe, fome 
will be projected at the fame inftant from the tibial, or 
any other veffel which may be opened. The representa¬ 
tion of a wave of blood, confiftingof two ounces, expelled 
by the contraction of the ventricle, and fuccelfively pal¬ 
ling through the various parts of the arterial fyftem, 
which proceeds on the fuppofition of the arteries being 
empty when the heart contracts, while in fact they are 
full, is calculated to convey a notion of the fubjeCt alto¬ 
gether erroneous. 
If an artery be opened far from the heart, each jet of 
blood will correfpond to the contraction of the ventricle, 
becaufe that contraction throws the blood from the ori¬ 
fice of the veffel. But, if this fluid were lent to the ex¬ 
tremities of the body by contractions of the arteries, the 
contracted and relaxed dates of thefe tubes fliould alter¬ 
nate with the corresponding conditions of the ventricle, 
and, confequently, each jet of arterial blood fliould take 
place when the ventricle is dilated, which is contrary to 
the faCt. 
In the common defcriptions of the circulation, we are 
informed that the heart, by its contraction, propels the 
blood into the aorta, w hich is then dilated; and that in 
the fucceeding period of time the aorta contracts to fend 
it on to all parts of the body. But this latter time does 
not exift at all. If a large artery be laid bare and ob- 
ferved, it will be feen to rife at each pulfation, but to un¬ 
dergo hardly any dilatation; nor does it contract to any 
greater degree. Contraction of the left ventricle, gene¬ 
ral movement of the whole mafs of arterial blood, and 
paflage of this blood into the capillary fyftem, are three 
circumftances which occur at the fame moment. “We 
may derive,” fays Bichat, “a very exaCt idea of the cir¬ 
culation by examining the mefenteric arteries of a living 
animal through the peritoneum. At each pulfation they 
appear to be raifed at once from their origin to their ex¬ 
tremities. No correCl notion of this fubjeft can be ac¬ 
quired by confidering the wave of blood as extending at 
each contraction through the arteries, and arriving luc- 
ceflively at various parts of the fyftem. All authors 
(peak at length of the progrefs of the arterial fluid, yet 
in no point is there more doubt and obfcurity, becaufe 
they all proceed on a falfe principle. It is not the frefh 
portion of blood expelled from the ventricle that enters 
the capillary fyftem at each contraction of the heart, but 
that which was neareft the capillaries in the arteries. 
Hence an interval of time feparates the departure of the 
blood from the heart from its arrival at the capillary fyf- 
tem, during which it remains, for a certain number of 
contractions, in the arteries. According to this mode 
of confidering the arterial part of the circulation, which 
is the only true and admiflible one, the windings of thefe 
veffels cannot produce any retardation. I regard, alfo, 
as entirely deftitute of foundation, all the remarks 
contained in phyfiological writers concerning the 
caufes of retardation in the courfe of the blood, occa¬ 
sioned, ift, by its paflage from a narrower into a wider 
fpace, and by the conical form of the arterial fyftem in 
general; 2dly, by friftion againlt the fides of the con¬ 
taining tubes; 3dly, by the angles; 4-thly, by the anafto- 
mofes, &c. All thefe remarks would hold good if the 
arteries were empty at the moment of contraction, be¬ 
came the blood would then really have a progreflive mo¬ 
tion ; but, in the genera! and inftantaneous (hock, which 
the mafs expanded through the whole fyftem experiences, 
thefe caufes are manifeltly inefficacious. I (till return to 
the trivial but illuftrating companion of the lyringe. 
Suppofe a tube (hould be adapted to it with a thouland 
turns, angles, and inequalities; if this tube and the fy¬ 
ringe are full, the depreftion of the pifton will caufe the 
water to flow out at the open end juft as foon as if the 
canal had been Ihort and ftraight. So true is it, that all 
the caufes of retardation which might affeCt the blood’s 
motion if the arteries were empty, do produce no fuch 
effeCt in the ordinary ftate of the veffels, that numerous 
judicious obfervers, who admitted the retardation in their 
reafonings, have feen in their experiments that the mo¬ 
tion was uniform throughout, and the fame in the 
branches as in the trunks. The pulfe occurs at the fame 
time in all parts of the arterial fyftem. How could this 
happen if the defcribed retardation actually took place ? 
The progrefs of phyfiology concerning the circulation 
has been much obitruCted by the current notions concern¬ 
ing the velocity of the blood’s motion in the arteries. Of 
this indeed we can form no true eftimate, becaufe the 
motion is not progreflive, and the blood does not flow, 
properly (peaking, but is puftied fuddenly by a gentle 
(hock, which cannot be fubje&ed to calculation.” 
When the contraction of the left ventricle propels the 
blood into the arteries, thefe tubes are (lightly dilated ; a 
general locomotion takes place, caufing the pulfe; and 
(ome red biood pafles into the general capillary fyftem. 
Thefe circumftances occur, when the fingerfeels the arte¬ 
rial pulfation at the diaftole of the tube. In the follow¬ 
ing inftant the ventricle is relaxed, that it may be filled 
again; the arteries, lefs full of blood, recover from the 
(light dilatation experienced juft before, and are reftored 
to their original fituation. This is the fyftole, defcribed 
as an aCtive, but in reality a merely paflive, ftate of the 
arterial tube. Since very little blood is thrown out of 
the ventricle at each pulfation, and, when the new quan¬ 
tity enters the arteries, fome paffes out at the oppofite 
end, tire arterial dilatation and the fubfequent contrac¬ 
tion are fo flight in degree as to be hardly obfervable. 
In this view, the arterial contraction does not take place 
in order to propel the blood; but it is a recovery of the 
veffel from its forced ftate, confequent on the paflage of 
the blood which diftended it, into the capillary fyftem. In 
Ihort, it is contraction from the ceffation of extennon. 
Although this reprefentation may feem to deftroy the 
fources of the varieties in the pulfe, obferved by authors, 
Bichat points out how the phenomena may (till be fuffi- 
ciently explained. “The caufes which may affeCt the 
pulfe, may relide, ift, in the heart, the foie agent of 
impulfion in their circulation. Its contraCtile power, 
augmented, diminifhed, or fympathetically modified, may 
caufe it, with the fame ftimulant, to aCt more quickly, or 
(lowly, or irregularly. Thus, organic affeCtions inevitably 
alter its aCfion. 2tlly. The blood may contain various 
natural or morbific matters capable of exciting the heart’s 
motion in various ways. 3d 1 y. The general capillary 
fyftem produces many variations in the pulfe, according 
as it receives a greater or lefs quantity of blood. There 
are, then, two circumftances to be particularly noted in 
the pulfe: ift, impulfion of the blood, a fudden and 
general motion of its mafs by the contraction of the heart; 
adly, locomotion of the arteries, an effeCt produced by 
this fluid on the arterial parietes. The firft is the molt 
effential point: the fecond would be changed, if the 
arterial tiffue, from the nature of which it arifes, were no 
longer the fame; it is not effential to the circulation. 
The obfcure vital properties of the arteries occaiions 
them to be the feat of very few affeCtions, either acute or 
chronic. Confequently, they can exert only a very (light 
fympathetic influence on other organs. For the fame 
reafons, other parts have very little Jympathetic influence 
on this fyftem. The innumerable fympathetic variations 
of the pulfe have their fource in the heart. 
The caufe of the motion of blood in the veins has 
never been accounted for. Hunter as we obferve has 
fuffered the fubjeCt to efcape his penetration. Bichat 
gives this obfcure account of it. “ The capillary fyftem, 
by its infenfible contraction, is conllantly pouring into 
the veins a certain quantity of blood. A general move¬ 
ment is communicated by this newly-arrived portion to 
the former contents of the velfeis ; and the addition at 
one end, as the fyftem is always full, renders it neceffary 
that 
