8, lungs;
9, pulmonary veins;
10, aorta;
11, alimentary canal;
12, liver;
13, hepatic artery;
14, portal vein;
15, hepatic vein.
]

All the veins of the body, except those from the lungs and the heart
itself, unite into two large veins, as already described, which pour their
contents into the right auricle of the heart, and thus the grand round of
circulation is continually maintained. This is called the systemic
circulation. The whole circuit of the blood is thus divided into two
portions, very distinct from each other.

191. The Portal Circulation. A certain part of the systemic or
greater circulation is often called the portal circulation, which
consists of the flow of the blood from the abdominal viscera through the
portal vein and liver to the hepatic vein. The blood brought to the
capillaries of the stomach, intestines, spleen, and pancreas is gathered
into veins which unite into a single trunk called the portal vein.
The blood, thus laden with certain products of digestion, is carried to
the liver by the portal vein, mingling with that supplied to the
capillaries of the same organ by the hepatic artery. From these
capillaries the blood is carried by small veins which unite into a large
trunk, the hepatic vein, which opens into the inferior vena cava. The
portal circulation is thus not an independent system, but forms a kind of
loop on the systemic circulation.

The lymph-current is in a sense a slow and stagnant side stream of
the blood circulation; for substances are constantly passing from the
blood-vessels into the lymph spaces, and returning, although after a
comparatively long interval, into the blood by the great lymphatic trunks.

Experiment 90. _To illustrate the action of the heart, and how it
pumps the blood in only one direction_. Take a Davidson or Household
rubber syringe. Sink the suction end into water, and press the bulb. As
you let the bulb expand, it fills with water; as you press it again, a
valve prevents the water from flowing back, and it is driven out in a
jet along the other pipe. The suction pipe represents the veins; the
bulb, the heart; and the tube end, out of which the water flows, the
arteries.

[NOTE. The heart is not nourished by the blood which passes through
it. The muscular substance of the heart itself is supplied with
nourishment by two little arteries called the _coronary arteries_,
which start from the aorta just