24 
PROCEEDINGS OP THE ACADEMY OP 
[Biolog. 
la order to understand this it is necessary to reflect a little while npon 
the peculiarities of the venous circulation in the portal system. From the 
resistance the blood in the general circulation encounters in traversing the 
capillaries, much of its moving force is lost, and to move regularly in the veins it 
requires some assistance, that is obtained chiefly from muscular movements and 
the aspiratory action of the movements of inspiration ; by neither of these is 
the flow in the portal reins affected. Besides, the portal veins possess no valves, 
and the blood contained therein is between two systems of capillaries, for these 
veins act as arteries in the liver, and the blood passes through a capillary net- 
work in that organ before it empties into the inferior vena cava. If in passing 
through ihe general capillary system, the blood loses nearly the whole of its 
moving force, it is evident that the capillary system in the liver must act in 
the same way, and with so much the more power, as that the tension of the 
blood in the portal vein is already itself less than ihat in the arteries. lu fact, 
well-known experiments, as for instance those made when the blood of the liver 
and that of the portal veins are collected for chemical examination, show that 
the circulation in the vena portarum is an entirely passive one, its current is di- 
rected towards the liver only on the one condition that there is a force to push 
it there, and this force comes from the abdominal walls, which, by pressing the 
mass of the intestines, propel the blood contained therein. Causes that may 
locally modify the circulation must, of course, act here with great efficacy, and 
it is far from being a matter of surprise that the haemorrhoidal veins, those 
most exposed, are often enlarged and diseased. Nature, as usual, has made 
provision for their protection, and were it not for the relief afforded them by 
the communications existing in the walls of the rectum between the inferior 
mesenteric vein, and the middle and hemorrhoidal branchesof the hypogastric, 
they would be still more often aflFected. 
It is very hard to say how far the rapidity of the current of the blood in the 
portal vein may be diminished, but one experiment is sufficient to show the in- 
fluence, in this respect, of causes constantly occurring. The ferro-cyanuret of 
potassium shows itself in the urine as soon as it reaches the general circula- 
tion ; this salt, when given 24 minutes after a meal appeared in the urine at 
the expiration of 16 minutes ; and when given 240 minutes after a meal, in 2 
minutes. In fact when the fatal consequences of the sudden injection into the 
general venous system of large quantities of even the most innocuous fluids is 
considered, the necessity of the prevention of the sudden passage into the infe- 
rior vena cava of the materials carried into the vena portarum, is at once ap- 
parent. Most important chemical changes undoubtedly occur in the liver, but 
it might be shown, we think, by reasons taken from numbers of facts in normal 
and pathological anatomy, both human and comparative, as also in physiology, 
that its action as a mechanical agent is most important. If a few ounces of water 
suddenly thrown into the general circulation of a dog can produce death, what 
would be the effect if, in man, the liver did not intervene between the inferior 
vena cava, and a barrel of lager beer? 
Now it being undoubtedly a fact that the pressure exerted by the abdom- 
inal muscles upon the mass of intestines is the cause of the onward movement 
of the blood in the portal veins towards the liver, any cause diminishing the 
pressure, whether position of the body, or feebleness of the muscles themselves, 
must lead to more or less stagnation of the blood, and cousequently to hemor- 
rhoidal affections. Tailors, shoemakers, dress-makers, and persons engaged in 
writing, which forces them to lean forward over a desk, are particularly sub- 
ject to haBmorrhoids ; and we would explain the well-known influence of seden- 
tary occupations generally upon this affection more by their producing relaxa- 
tion of the anterior abdominal walls, than by the deficiency of exercise and the 
constipation attending them, though these must play their part also. An un- 
doubted cause of an attack of haemorrhoids, and one not unfrequently witnessed, 
is parturition. It is not very uncommon for the patient to suffer most severely 
[Dec. 
