branches, L M, and the brachial plexus of nerves, I. Both these
triangles and their contents are completely sheathed by that thin
scarf-like muscle, named platysma myoides, A A, Plate 3, the fibres of
which traverse the neck slantingly in a line, O A, of diagonal direction
opposite to and secant of that of the sterno-mastoid muscle.

When the skin and subcutaneous adipose membrane are removed by careful
dissection from the cervical region, certain structures are exposed,
which, even in the undissected neck, projected on the superficies, and
are the unerring guides to the localities of the blood-vessels and
nerves, &c. In Plate 4, the top of the sternum, 6; the clavicle, 7; the
"Pomum Adami," 1; the lower maxilla at V; the hyoid bone, Z; the
sterno-cleido-mastoid muscle, B; and the clavicular portion of the
trapezius muscle, 8; will readily be felt or otherwise recognised
through the skin, &c. When these several points are well considered in
their relation to one another, they will correctly determine the
relative locality of those structures--the blood-vessels, nerves, &c.,
which mainly concern the surgical operation.

The middle point, between 7, the clavicle, and 6, the sternum, of Plate
4, is marked by a small triangular space occurring between the
clavicular and sternal divisions of the sterno-cleido-mastoid muscle.
This space marks the situation (very generally) of the bifurcation of
the innominate artery into the subclavian and common carotid arteries of
the right side; a penetrating instrument would, if passed into this
space at an inch depth, pierce first the root of the internal jugular
vein, and under it, but somewhat internal, the root of either of these
great arterial vessels, and would wound the right vagus nerve, as it
traverses this region. For some extent after the subclavian and carotid
vessels separate from their main common trunk, they lie concealed
beneath the sterno-mastoid muscle, B, Plate 4, and still deeper beneath
the sternal origins of the sterno-hyoid muscle, 5, and sterno-thyroid
muscle, some of whose fibres are traceable at the intervals. The
omo-hyoid muscle and the deep cervical fascia, as will be presently seen,
conceal these vessels also.

The subclavian artery, A, Plate 4, first appears superficial to the
above-named muscles of the cervical region just at the point where,
passing from behind the scalenus muscle, N, Plate 4, which also conceals
it, it sinks behind the clavicle. The e