xact locality of the artery in
this part of its course would be indicated by a finger's breadth
external to the clavicular attachment of the sterno-mastoid muscle. The
artery passes beneath the clavicle at the middle of this bone, a point
which is indicated in most subjects by that cellular interval occurring
between the clavicular origins of the deltoid and great pectoral
muscles.

The posterior cervical triangle, 9, 8, 7, Plate 4, in which the
subclavian artery is situated, is again subdivided by the muscle
omo-hyoid into two lesser regions, each of which assumes somewhat of a
triangular shape. The lower one of these embraces the vessel, A, and
those nerves of the brachial plexus, I, which are in contact with it.
The posterior belly of the omo-hyoid muscle, K, and the anterior
scalenus muscle, N, form the sides and apex of this lesser triangular
space, while the horizontal clavicle forms its base. This region of the
subclavian artery is well defined in the necks of most subjects,
especially when the muscles are put in action. In lean but muscular
bodies, it is possible to feel the projection of the anterior scalenus
muscle under the skin, external to the sterno-mastoid. The form of the
omo-hyoid is also to be distinguished in the like bodies. But in all
subjects may be readily recognised that hollow which occurs above the
clavicle, and between the trapezius, 8, and the sterno cleido-mastoid, 7
B, in the centre of which hollow the artery lies.

The contents of the larger posterior cervical triangle, formed by B, the
sterno-mastoid before; 9, the splenius; and 8, the trapezius behind,
and by the clavicle below, are the following mentioned structures--viz.,
A, the subclavian artery, in the third part of its course, as it emerges
from behind N, the scalenus anticus; L, the transversalis colli artery,
a branch of the thyroid axis, which will be found to cross the
subclavian vessel at this region; I, the brachial plexus of nerves,
which lie external to and above the vessel; H, the external jugular
vein, which sometimes, in conjunction with a plexus of veins coming from
behind the trapezius muscle, entirely conceals the artery; M, the
posterior scapular artery, a branch of the subclavian, given off from
the vessel after it has passed from behind the scalenus muscle; O,
numerous lymphatic glands; P, superficial descending branches of the
cervical plexus of nerves; and Q, ascending superficial branches of the
same plexus. Al