The lateral aspect of the neck is divided by the sterno-mastoid muscle into two triangles—anterior and posterior. The anterior triangle is bounded in front by the middle line of the neck, behind by the anterior border of the sterno-mastoid muscle, and above by the lower border of the ramus of the jaw.
The space so marked out is divided into three smaller triangles by the digastric muscle and by the anterior belly of the omo-hyoid:
(1) The submaxillary triangle, above the digastric muscle, containing the submaxillary gland. (Fig. IV, 17.)
(2) The muscular triangle, anterior to the omo-hyoid muscle. (Fig. IV, 19.)
The carotid triangle, bounded above by the posterior belly of the digastric, behind by the anterior (Fig. IV, 16.) border of the sterno-mastoid muscle, and in front by the anterior belly of the omo-hyoid. In this triangle the common carotid bifurcates, and the external carotid gives off most of its branches.
The posterior triangle is bounded in front by the posterior border of the sterno-mastoid, behind by the anterior border of the trapezius, and below by the middle third or fourth of the clavicle. (Fig. IV, 15,18.) The triangle is subdivided by the posterior belly of the omo-hyoid, which cuts off the small subclavian triangle below from the more extensive occipital triangle above.