The axillary sheath is an extension of this fascia around the subclan aery and brachial plexus. Fascia around nerve bundle of brachial plexus is derived from:
The brachial plexus and the subclan aery course between the anterior and middle scalene muscles, and as the nerve plexus and aery emerge from those muscles, they carry an extension of the preveebral fascia along to form the axillary sheath. Ref: Moon D.A., Foreman K.B., Albeine K.H. (2011). Chapter 26. Triangles and Root of the Neck. In D.A. Moon, K.B. Foreman, K.H. Albeine (Eds), The Big Picture: Gross Anatomy.
Preveebral fascia
Pretracheal fascia
Investing layer
Superficial cervical fascia