Take a point midway along the posterior border of the sterno-mastoid muscle, and from this point draw three lines:
1. Upwards towards the lobe of the ear = the great auricular nerve (2 and 3 C).
2. Upwards along the posterior border of the sterno-mastoid muscle = small occipital nerve (2 C).
3. Forwards towards the middle line of the neck = the transverse cervical nerve (2 and 3 C).
By producing these three lines in a downward direction, the descending branches of the plexus are roughly indicated.
Thus the great auricular produced = the supraclavicular nerve; the small occipital produced = the suprasternal nerve; the transverse cervical produced = the supra-acromial nerve. The three descending trunks are derived from the third and fourth cervical nerves, and all the branches of this plexus arise from the anterior primary divisions of their respective nerves.
Draw a line from a point midway between the tip of the mastoid process and the angle of the jaw to the middle of the posterior border of the sterno-mastoid muscle, and thence across the posterior triangle to the anterior border of trapezius.