cle, &c., are performed in these vessels.

[Illustration: Fig. 72. An ideal view of a portion of the pulmonic
circulation. 1, 1, A branch of the artery that carries the impure blood
to the lungs. 3, 3, Capillary vessels. 2, 2, A vein through which red
blood is returned to the left side of the heart.]

[Illustration: Fig. 73. An ideal view of a portion of the systemic
circulation. 1, 1, A branch of the aorta. This terminates in the
capillaries, (3, 3.) 2, 2, A vein through which the impure blood is
carried to the right side of the heart.]

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350. What do the capillaries constitute? For what are they remarkable?
351. What relation do they bear to the arteries and veins? What
important operations are performed in these vessels? What is
represented by fig. 72? By fig. 73?

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CHAPTER XVIII.

PHYSIOLOGY OF THE CIRCULATORY ORGANS.

352. The walls of all the cavities of the heart are composed of
muscular fibres, which are endowed with the property of contracting
and relaxing, like the muscles of the extremities. The contraction and
relaxation of the muscular tissue of the heart, produce a diminution
and enlargement of both auricular and ventricular cavities. The
auricles contract and dilate simultaneously, and so do the ventricles;
yet the contraction and dilatation of the auricles do not alternate
with the contraction and dilatation of the ventricles, as the
dilatation of the one is not completed before the contraction of the
other commences. The dilatation of the ventricles is termed the
_di-as'to-le_ of the heart; their contraction, its _sys'to-le_.

353. The ventricles contract quicker and more forcibly than the
auricles, and they are three times longer in dilating than contracting.
The walls of the right ventricle, being thinner than the left, are more
distensible, and thus this cavity will contain a greater amount of
blood. This arrangement adapts it to the venous system, which is
more capacious than the arterial. The thicker and more powerful walls of
the left ventricle adapt it to expel the blood to a greater distance.

354. The valves in the heart permit the blood to flow from the
auricles to the ventricles, but prevent its reflowing. The valves at
the commencement of the aorta and pulmonary artery, permit the blood
to flow from the ventricles into these vessels, but prevent its
returning.

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352-366. _Give the physiol