6 ro ANATOMY. 
lerial fyftem. That the blood, again, in the arteries, flows 
from tire heart towards the extreme parts of the body, is 
proved by the microfcope, and by a ligature cn the artery 
of a living animal, and likewife by the fabric, mechanilm, 
and proportion, of the femilunar valves between the ar¬ 
teries and their correfponding ventricles. Whatever ar¬ 
tery is flopped by a ligature, the fwelling enfues in that 
part between the heart and the ligature, whilft the other 
part beyond the ligature, which is more remote from the 
heart, is emptied. Neither has it there any puliation, nor, 
if it be there wounded, will it yield any blood. The fame 
effedts which we fee follow from a ligature, are likewife 
often produced by difeafe ; as when fonre tumour, by com- 
preflure or an aneurifm, intercepts the blood’s motion from 
the heart. Experiments of this kind have been made on 
mcft of the arteries; anaftomofes, however, or the blood 
flowing through a neighbouring branch, or the retrocef- 
flon of the blood in a dying animal, form exceptions to 
this rule. 
But the courfe or motion of the venous blood has been 
always more doubted; almoft all the ancients have been 
perfuaded, that the blood in the veins flowed through them, 
either from the heart or from the liver, to all parts of the 
body. Very few of them knew that this was an error. 
Several of them have, indeed, acknowledged it to befalfe 
fn the pulmonary vein. But that the blood did not move 
from the heart in the vena cava, was known to flill fewer 
anatomifts among the ancients. Dr. William Harvey was 
the firft who experimentally aflerted the motion of the 
blood returning in the veins to the heart, in fuch a man¬ 
ner as to render the whole intelligible, and leave no room 
for doubt. The valves of the veins lead us to this truth. 
For the common ufe or office of thefe valves is, to deter¬ 
mine the prelfure that is made from any quarter upon the 
veins, towards the heart, by allowing no opportunity to 
the venous blood that has once entered the trunk to flow 
back to the branches. For, fince the valves open upwards 
towards the heart, the blood enters and expands them; 
and thofe parts of the valves which project into the cavity 
of the vein, approach towards the axis, until the oppollte 
fides, by meeting together, (hut up the tube. Another 
office of the valves in the veins feems to be for fuflaining 
the weight of the blood, that its upper columns may not 
gravitate upon the lower; nor the blood, flowing through 
the trunk, make too great a refiftance againft that which 
follows it through the branches. For if, from the flower 
motion of the blood, its weight or prelfure fliall, in any 
part, much exceed the impufe that drives it on, fo as to 
caufe fome part of the column to defeend by its weight, 
it is, in that cafe, immediately catched, and fuflained in its 
relapfe by the next adjacent valve, which hinders it from 
urging againft the next fucceeding column, and affords time 
and opportunity for fome contiguous mufcle, by its pref- 
fure or concuflion, to propel the column. This is the rea- 
fon why valves, are placed in the veins of the limbs and 
necks, in which parts they are both more numerous and 
more robuft than ell'ewhere. Moreover, the valves placed 
in the right fide of the heart are fo conftrudted, that they 
freely permit blood, air, or wax, to pafs from the venous 
trunks of the cava into the heart, but deny any reflux from 
the heart into the veins. Again, ligatures, in a living per- 
fon, make this circumftance more evident. When the 
veins of the limbs are tied, either by defign or accident, 
about the hams, arms, ancles, or wrifts, the limb below 
the ligature fwells, the veins fill and diftend themfelves, 
and, when opened, make a free difcharge of blood. But 
at the fame time nothing of this kind happens above the 
ligature, nor any of the-veins to be feen there. Thefe li¬ 
gatures will ferve to keep the blood in any limb round 
which they are. tied, that it may not return to the heart, 
and be loft through a wound in another part. 
The experiments'which have been made in living ani¬ 
mals, to prove this courfe of the blood, are ftill more ac¬ 
curate. From them, it appears, that by tying any vein, 
in a living animal, near the cava, or belonging to the pul¬ 
monary veins, that part always fwells which is moil remote 
From the heart, all below the ligature appearing diftended 
with the retained blood, while above and next the heart 
they are pale and flaccid. Laftly, if the arteries are tied 
at the fame time with the veins, thefe laft remain flaccid 
and empty; but, upon removing the ligature from the ar¬ 
teries, the veins are immediately filled. In like manner, 
the infufion of poifons or medicinal liquors fliew, that, in¬ 
to whatever vein you inje£t chemical acid fpirits, the 
force of the poifon is driven along with the blood to the 
heart itfelf. That the brain is affedled with the narcotic 
virtue of opium, and the inteftines and ftontach 'with the 
virtue of purgatives and emetics injected into the veins, is 
a demonftration that the blood, with which thefe fubftan- 
ces were mixed, had pafled through the ramifications of 
the veins to the heart, and from thence through the whole 
body. We have another proof in the transfufions of 
blood; in which all the blood from the arteries of one 
animal is urged into the veins of another exhaufted of 
blood; whereby the heart, arteries, and empty veins, of 
the latter, become fo turgid, and well repleniffied, that 
they produce a remarkable degree of vivacity in the ani¬ 
mal, or even caufe it to labour under a plethora. 
That the blood pafles from the leaft arteries into the leaft 
veins, we are clearly taught by anatomical injcElion where, 
by one arterial trunk, we eafily fill all the arteries and 
veins, almoft throughout the whole body, provided the 
liquor be very fluid, fo as to pafs eafily into the veffels 
of the head, mefentery, heart, and lungs. The circula¬ 
tion of the blood is therefore now received by every one 
as a medical truth ; namely, that all the blood of the hu¬ 
man body is carried through the aorta, from the left ca¬ 
vity of the heart, to the extreme parts or converging ends 
of the arterial branches; from whence the whole mafs is 
again tranfmitted into the leaft veins, which convey it to 
the larger, and from them into the cava and heart itfeif; 
in which collide it perpetually goes and returns during life. 
The courfe of the humours in the lymphatic veins which 
have valves, appears both from the nature of thofe veins, 
and from ligatures. For every lymphatic veins, when 
tied, fwells between its fmaller extremities and the thora¬ 
cic duff ; but grows flaccid between the duft and the li¬ 
gature. All the valves in thefe, like thofe- of the blood- 
veins, give a free paflage for flatus and mercury to flow 
to the thoracic dudt. But they make a refiftance, and of¬ 
ten an obftinate one, to any return the other way ; although 
fometinies they have been known to yield. The vapours 
that moiften the whole cellular fubftance, the fteants of 
the abdomen and other cavities, are all abforded by the 
leaft pellucid veins, and fo conveyed to the blood-veins, 
that their contained juices may pafs on to the heart. And 
from thence it is that oedema enfues, when a vein is com- 
prefled by a ligature; becaufe, by intercepting the courfe 
of the absorbing veins by the ligature, the vapours, being 
unabforbed, flagnate. All the juices, therefore, in the hu¬ 
man body, are driven out of the heart into the aorta; 
from whence they are all uniformly returned again to the 
heart, by the veins. To complete this circle, it only re¬ 
mains for us to find out a courfe for the blood from the right 
to the left cavities of the heart; but this firft fuppofes us 
to be acquainted with the hiftory of the lungs and-the pul¬ 
monary veifels. 
OF THE LUNGS. 
The lungs are two large fpongy fubftances, of areddiftt 
colour in children, greyifli in adult fubjeifts, andbluifhin 
old age. They fill the whole cavity of the thorax, one 
being leated in the right fide, tiie other in the left, and 
are parted by the mediaftinum and heart. They are con¬ 
vex next the ribs, concave next the diaphragm, and irre¬ 
gularly flatted and deprefled next the mediaftinum and 
heart. They are diftinguilhed into the ri J.ht and left lung; 
and each of thefe into two or three portions called loin, of 
which the right lung has commonly three, and the left 
lung two. At the lower edge of the left lung, there is an 
3, indented 
