The connection between arteries and veins is shown only in the brain.

Both heart and blood vessels are considerably enlarged to show clearly the course of the blood.] In passing through the liver, the blood is purified of some irritating substances picked up from the food-tube, and the melted food which it contains is further prepared for the use of the cells of the body.

The portal vein of the liver breaks up into a network of veins, and these again break up into a number of tiny capillaries, in which the blood is acted upon by the cells of the liver.

These capillaries gather together again to form veins, and finally unite into two large veins at the back of the liver, which run directly into the great trunk-pipe of all the veins of the body--the _vena cava_ (or "empty vein," so called because it is always found empty after death), about an inch from where this opens into the right side of the heart.
In the vena cava the blood from the food-tube, rich in food, but poor in oxygen, mixes with the impure, or used-up, blood brought back by the veins from all over the body and, passing into the right side of the heart, is pumped by the heart through a large blood-pipe to the lungs.
This large blood-pipe divides into two branches, one for each lung; and these again break up into smaller branches, and finally into tiny capillaries, which are looped about in fine meshes, or networks, around the air-cells of the lung.

Here, through the thin and delicate walls of the capillaries the blood cells give off, or breathe out, their carbon dioxid and other waste gases (which are passed out with our outgoing breath), and at the same time they breathe in oxygen which our incoming breath has drawn into the lungs.
This oxygen is picked up by, and combines with, the red coloring matter of the millions of little oxygen sponges, or baskets--the red corpuscles--and turns them a light red color, causing the blood to become bright red, such as runs in the arteries and is known as _arterial blood_.
The loops of tiny capillaries around the air cells of the lungs run together again to form larger pipes; and these unite, at the point of each lung nearest the heart, to form two large blood pipes--one from each lung--which pour the rich, pure blood, loaded with both food and oxygen into the left side of the heart.