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OF THE DIVISIONS OF THE HUMAN BODY. 47
surface of the body.
Around each of these minute
cells is a network of arterial and venous capillaries, and
it is through the coats of these that the air acts upon
the blood, giving it oxygen, and receiving from it car
bon. There enters, then, into the structure of the lungs,
the pleura, or external membrane ; the air-tubes and
vesicles; the arteries, the veins, the lymphatics, the
nerves, and the areolar tissue, which holds them all to
gether.
All the blood passes through the lungs, to be brought
into contact with the atmosphere ; their animal mem
branes forming no barrier to the chemical action of
gases. This contact of the air with the circulating fluids
is necessary to all —to organized beings vegetables and
animals. In vegetables this contact takes place in the
leaves, in fishes by the gills, in the higher animals by
lungs.
OF THE LIVER.
The size of an organ is some measure of its import
ance. The liver is an irregular-shaped, brown mass,
weighing four pounds in health, but often much en
larged in disease. It lies on the right side of the ab
the stomach, and
domen, under the diaphragm, opposite
partly covered by
the short ribs. The liver is a collec
tion of a vast number of glands, each of which separates
the bile from the blood. The blood thus purified is the
venous blood gathered from the stomach and intestines,
and which contains a portion of the nutritive matter.
All these veins gather into one common vein, the vena
porta, which enters and branches out in the liver into
minute vessels ; the purified blood collects in another