When it encroaches upon the posterior triangle the scalene muscles are attached to it, and the subclavian artery and the lower trunk and medial cord of the brachial plexus pass over it in a groove behind the scalenus anterior.
"Manual of Surgery Volume Second: Extremities--Head--Neck. Sixth Edition."
Alexander Miles Alexis Thomson
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.
"Surgical Anatomy"
Joseph Maclise
N. scalenus anticus muscle.
"Surgical Anatomy"
Joseph Maclise