eds from the left ventricle of the heart, and
contains the pure, or nutrient blood. This trunk gives off branches,
which divide and subdivide to their ultimate ramifications,
constituting the great arterial tree which pervades, by its minute
subdivisions, every part of the animal frame. This great artery and
its divisions, with their returning veins, constitute the _greater_,
or _systemic circulation_.

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What does this artery and its corresponding veins establish? Explain
fig. 69. 345. Describe the aorta. What do this artery and its
corresponding veins constitute?

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[Illustration: Fig. 70. The aorta and its branches. 1, The commencement
of the aorta. 2, The arch of the aorta. 3, The carotid artery. 4, The
temporal artery. 5, The subclavian artery. 6, The axillary artery. 7, The
brachial artery. 8, The radial artery. 9, The ulnar artery. 10, The iliac
artery. 11, The femoral artery. 12, The tibial artery, 13. The peroneal
artery.]

346. The VEINS are the vessels which return the blood to the auricles
of the heart, after it has been circulated by the arteries through
the various tissues of the body. They are thinner and more delicate in
structure than the arteries, so that when emptied of their blood, they
become flattened and collapsed. The veins commence by minute radicles
in the capillaries, which are every where distributed through the
textures of the body, and coalesce to constitute larger and larger
branches, till they terminate in the large trunks which convey the
dark-colored blood directly to the heart. In diameter they are much
larger than the arteries, and, like those vessels, their combined area
would constitute an imaginary cone, the apex of which is placed at the
heart, and the base at the surface of the body.

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What does fig. 70 represent? 346. What are the veins?

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347. The communications between the veins are more frequent than
between the arteries, and take place between the larger as well as
among the smaller vessels. The office of these inosculations is very
apparent, as tending to obviate the obstructions to which the veins
are peculiarly liable, from the thinness of their coats, and from
inability to overcome great impediments by the force of their current.
These tubes, as well as the arteries, are supplied with nutrient
vessels, and it is to be presumed that nervous filaments from