[Illustration]
173. The efforts resorted to by nature for the suppression of serious
hemorrhages depend on the capabilities of the arteries as resulting
from their structure, into which it becomes an object of importance
minutely to inquire. With this view, the old division of an artery
into three coats may be continued, the difference between ancient and
modern anatomy being in their subdivision into different textures or
layers. The annexed diagram shows the edge of a large artery, which
has been divided circularly, and magnified so as to exhibit six layers
in a distinct manner; each of the three ancient coats is divided into
two. The _inner_ or old serous coat is shown to be separable into
_two_: the epithelial, marked 1, and the fenestrated, marked 2. The
_middle_ coat is also separated into _two_: the inner, or _muscular_,
marked 3, and the outer, or _elastic_, marked 4. The _outer_ coat is
divisible also into two layers, the _inner_, marked 5, and the _outer_,
marked 6; number 5 being composed more of elastic fibers: number 6 more
of areolar fibers, by which tissue, in a less condensed state, the
arteries of the extremities are attached to their sheaths. Such may
be considered to be the general composition of a large artery, each
particular structure remaining to be examined.
[Illustration: No. 1.