a colorless, thin, transparent membrane, yet so strong that it can, it
is thought, better resist a powerful pressure than either of the others.
Arteries are very elastic as well as extensible, and their chief
extensibility is in length. If an artery of a dead body be divided,
although empty, its cylindrical form will be preserved.

The _Veins_ are the vessels through which the venous blood returns to
the auricles of the heart. They are more numerous than the arteries, and
originate from numerous capillary tubes, while the arteries are given
off from main trunks. In some parts of the body, the veins correspond in
number to the arteries; while in others, there are two veins to every
artery. The veins commence by minute roots in the capillaries, which are
everywhere distributed through the body, and gradually increase in size,
until they unite and become large trunks, conveying the dark blood to
the heart. The veins, like the arteries, have three coats. The external,
or cellular coat, resembles that of the arteries; the middle is fibrous,
but thinner than the corresponding one of the arteries; and the internal
coat is serous, and analogous to that of those vessels. The veins belong
to the three following classes: (1.) The systemic veins, which bring the
blood from different parts of the body and discharge it into the vena
cava, by means of which it is conveyed to the heart; (2), the pulmonary
veins, which bring the arterial, or bright red blood from the lungs and
carry it to the left auricle; (3), the veins of the portal system, which
originate in the capillaries of the abdominal organs, then converge into
trunks and enter the liver, to branch off again into divisions and
subdivisions of the minutest character.

The _Capillaries_ form an extremely fine net-work, and are distributed
to every part of the body. They vary in diameter from 1/3500 to 1/2000
of an inch. They are so universally prevalent throughout the skin, that