he capillary vessels, and the
effect of the union of the branches of the veins is to accelerate the
speed of the blood as it returns from the capillary vessels to the
venous trunks.

In the smaller vessels a frequent running together, or anastomosis,
occurs. This admits of a free communication between the currents of
blood, and must tend to promote equability of distribution and of
pressure, and to obviate the effects of local interruption. The arteries
are highly elastic, being extensile and retractile both in length and
breadth. During life they are also contractile, being provided with
muscular tissue. When cut across they present, although empty, an open
orifice; the veins, on the other hand, collapse.

In most parts of the body the arteries are inclosed in a sheath formed
of connective tissue, but are connected so loosely that, when the vessel
is cut across, its ends readily retract some distance within the sheath.
Independently of this sheath, arteries are usually described as being
formed of three coats, named, from the relative positions, external,
middle, and internal. This applies to their structure so far as it is
discernible by the naked eye. The internal, serous, or tunica intima, is
the thinnest, and is continuous with the lining membrane of the heart.
It is made up of two layers--an inner, consisting of a layer of
epithelial scales, and an outer, transparent, whitish, highly elastic,
and perforated. The middle coat, tunica media, is elastic, dense, and
of a yellow color, consisting of nonstriated muscular and elastic
fibers, thickest in the largest arteries and becoming thinner in the
smaller. In the smallest vessels it is almost entirely muscular. The
external coat, tunica adventitia, is composed mainly of fine and closely
woven bundles of white connective tissue, which chiefly run diagonally
or obliquely around the vessel. In this coat the nutrient vessels, the
vasa vasorum, form a capillary network, from which a few penetrate as
far as the muscular coat.

The veins differ from arteries in possessing thinner walls, less elastic
and muscular tissue, and for the most part a stronger tunica adventitia.
They collapse when cut across or when they are empty. The majority of
veins are provided with valves; these are folds of the lining membrane,
strengthened by fibrous tissue. They favor the course of the blood and
prevent its reflux. The nerves which supply both the arteries and the
veins come from the sym