ts sensibility? How can an idea of the
structure of the heart be obtained?

-=-=-=-=-=-=-=-=-=-=-=-=

341. The ARTERIES are the cylindrical tubes that convey the blood from
the heart to every part of the system. They are dense in structure,
and preserve, for the most part, the cylindrical form, when emptied of
their blood, which is their condition after death.

342. The arteries are composed of three coats. The external, or
cellular coat, is firm and strong; the middle, or fibrous coat, is
composed of yellowish fibres. This coat is elastic, fragile, and
thicker than the external coat. Its elasticity enables the vessel to
accommodate itself to the quantity of blood it may contain. The
internal coat is a thin, serous membrane, which lines the interior of
the artery, and gives it the smooth polish which that surface
presents. It is continuous with the lining membrane of the heart.

343. Communications between arteries are free and numerous. They
increase in frequency with diminution in the size of the branches, so
that through the medium of the minute ramifications, the entire body
may be considered as one circle of inosculation. The arteries, in
their distribution through the body, are enclosed in a loose, cellular
investment, called a sheath, which separates them from the surrounding
tissues.

344. The PULMONARY ARTERY commences in front of the origin of the
aorta. It ascends obliquely to the under surface of the arch of the
aorta, where it divides into two branches, one of which passes to the
right, the other to the left lung. These divide and subdivide in the
structure of the lungs, and terminate in the capillary vessels, which
form a net-work around the air-cells, and become continuous with the
minute branches of the pulmonary veins. This artery conveys the impure
blood to the lungs, and, with its corresponding veins, establishes the
_lesser_, or _pulmonic circulation_.

-=-=-=-=-=-=-=-=-=-=-=-=

341. What are arteries? 342. Give their structure. 343. What is said
of the communications between the arteries? In their distribution, how
are they separated from the surrounding tissues? 344. Describe the
pulmonary artery.

-=-=-=-=-=-=-=-=-=-=-=-=

[Illustration: The divisions of this artery continue to divide and
subdivide, until they become no larger than hairs in size. These minute
vessels pass over the air-cells, represented by small dark points around
the margin of the lungs.]

345. The AORTA proce