Encyclopædia americana. A popular dictionary of arts, sciences, literature, history, politics and biography, a new ed.; including a copious collection of original articles in American biography; on the basis of the 7th ed. of the German Conversations-lexicon. Ed. by Francis Lieber, assisted by E. Wigglesworth ...

BLOOD —BLOOD-VESSELS. ~139
condition of life. With it, except in cases internal lining of the arteries, although
of fainting, suffocation, &c., life ceases. belonging to different classes of memThe heart, the centre of the circulation branes, are both very thin and soft. The
of the -blood, has a two-fold motion, of second coat is very thick, tough and elascontraction and dilatation,which constant- tic, being that which chiefly gives their
ly alternate. With the heart two kinds of peculiar appearance to the arteries. The
vessels are connected-the arteries and the third is formed of fibres, apparently musveins. (See Blood-Vessels.) The circula- cular, arranged in circular rings around
tion of the blood proceeds with an aston- the tube of the vessels. It is well known
ishing rapidity: did it flow at an equal that the pulse of the heart is felt in the
rate in a straight line, it would. run, in the arteries alone, although, in the bleeding
space of one minute, through 149 feet. of a vein, we sometimes see the blood
This swiftness, however, exists only in start as if in unison with the beating of'
the larger vessels near the heart; the far- the heart. The pulse is produced by the
ther the blood recedes from the heart, the wave or stream of blood, which is driven
-slower its motion becomes. In a grown- *by the heart through ththe arteries, disup person, in good health, we may reck- tending and slightly elevating them, after
on the mass of blood at 24-30 pounds. which they instantly contract from their
Blood-Vessels are the tubes or vessels elasticity, and thus force the blood into
in which the blood circulates. They are the smaller vessels. The pulse varies in
divided into two classes,-arteries and its character with the general state of the
veins,-which have two points of union health. (See Pulse.) When arteries are
or connexion-the first in the heart, from cut or wounded, the firmness of their
which they both originate, and the other coats prevents their closing, and hence
in the minute vessels or net-work, in arises the fatal nature of wounds of large
which they terminate. The arteries arise vessels, which will remain open till they
from the heart, and convey the blood to are tied up, or till death is produced.all parts of the body; the veins return it The veins commence in small capillary
to the heart.'The arteries distribute tubes in every part of the body, and, by
throughout the body a pure, red blood, their gradual union, form large trunks,
for the purposes of nourishment; while till they at last terminate in two (one
the veins return to the heart a dark-col- ascending from the lower parts of the
ored blood, more or less loaded with im- body, the other descending from the head
purities, aned deprived of some of its valu- and arms), which pour their contents into
able properties. But this is not returned the heart. Their structure is much less
again to the body in the same state. For firm than that of the arteries. They
the heart is wisely divided into two por- are very thin and soft, consisting of only
tions or sides, a right and left, one of two thin coats or membranes. The inner,
which receives the impure blood from or lining membrane, is frequently doubled
the veins, and sends it to the lungs to be into folds, forming valves, which nearly
defecated and freshly supplied with oxy- close the passage in the veins, and thus
gen or vital air, while the other receives give very material support to the blood
the pure red blood finom the lungs, and as it is moving up in them towards the
circulates it anew through the arteries. heart. These valves are not found in the
The arteries arise from the left ventricle veins of the bowels, the lungs or the
of the heart by one large trunk, nearly an head. The number of the veins is much
inch in diameter, which is gradually sub- greater than that of the arteries, an artery
divided into smaller ones, as it proceeds being often accompanied by two veins.
towards the limbs, till they terminate, at They differ also in this, that, while the
last, in vessels so small as to be almost arteries are deeply seated in the flesh, to
invisible, and in a fine net-work of cells, guard them from injury, the veins are
extending through the whole body, in very firequently superficial, and covered
which the blood is poured out, and nutri- only by the skin. The veins, it is well
tion or the increase of the body takes known, are the vessels commonly opened
place, and from which the residue is in blood-letting, although, in cases which
taken up by the small veins, to be re- render it necessary, a small artery is
turned to the heart. The arteries and sometimes divided.-There are two porveins are widely different in their struct- tions of the venous system, which do not
ure, as well as their uses. The former correspond exactly with our genelal de
are composed of very strong, firm, elastic scription; these are the veins of the bowcoats or membranes, which are four in els and of the lungs. The former circunumber. The external covering and the late their blood through the liver before