Final answer:
The brachial plexus and subclavian artery are located between the anterior and middle scalene muscles, with the middle scalene being the longest of the scalene muscles.
Step-by-step explanation:
The area between the anterior and middle scalene muscles is clinically significant because it is where the brachial plexus and the subclavian artery pass through. The middle scalene, known as the longest scalene muscle, is situated between the anterior scalene and the posterior scalene.
Regarding the plexus and artery between these muscles, the brachial plexus (comprising spinal nerves C4 through T1) extends between the anterior and middle scalene muscles, while the subclavian artery also travels through this space heading towards the arm. This anatomical region is important for both nervous system communication to the upper limb and the vascular supply.