Fascia around nerve bundle of brachial plexus is derived from

The muscular floor of the posterior triangle is covered by preveebral layer of deep cervical fascia which forms the fascial carpet of the floor of posterior triangle.The axillary sheath around subclan aery and brachial plexustravelling from root of neck to the upper limb.pus collected in the posterior triangle deep to its fascial carpet from tubercular cervical veebrae may track downwards and laterally along the axillary sheath to first appear in the axilla or even in the arm subsequently.Reference: Textbook of anatomy, Upper limb, and thorax, Vishram Singh, 2nd edition, page no.80