the
sympathetic nerves are distributed to their coats.

348. The external, or cellular coat of the veins, is dense and firm,
resembling the cellular tunic of the arteries. The middle coat is
fibrous, like that of the arteries, but extremely thin. The internal
coat is serous, and also similar to that of the arteries. It is
continuous with the lining membrane of the heart at one extremity, and
with the lining membrane of the capillaries at the other.

349. At certain intervals, the internal coat forms folds, or
duplicatures, which constitute valves. They are generally composed of
two semilunar folds, one on each side of the vessel. The free
extremity of the valvular folds is concave, and directed forward, so
that while the current of blood sets toward the heart, they present no
impediment to its free passage; but let the current become retrograde,
and it is impeded by their distention. The valves are most numerous in
the veins of the extremities, particularly the deeper veins situated
between the muscles; but in some of the larger trunks, and also in
some of the smaller veins, no valves exist.

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Where do they commence? 347. What is said of their communications?
What is the apparent design of the inosculations of the veins? What
vessels are distributed to the coats of the veins? 348. Give the
structure of the coats of the veins. 349. How are the valves in the
veins formed?

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[Illustration: Fig. 71. A vein laid open to show the valves. 1, The trunk
of the vein. 2, 2, Its valves. 3, An opening of a branch into the main
trunk.]

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What is their use? Where are they the most numerous?

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350. The CAPILLARIES constitute a microscopic net-work, and are so
distributed through every part of the body as to render it impossible
to introduce the smallest needle beneath the skin, without wounding
several of these fine vessels. They are remarkable for the uniformity
of diameter, and for the constant divisions and communications which
take place between them.

351. The capillaries inosculate, on the one hand, with the terminal
extremity of the arteries, and on the other, with the commencement of
the veins. They establish the communication between the termination of
the arteries and the beginning of the veins. The important operations
of secretion and the conversion of the nutrient materials of the blood
into bone, mus