The following is true about scalenus anterior muscle -

Scalenus anterior origins from anterior tubercles of the transverse process of four typical cervical vertebrae (i.e. C3-C6) and insert on scalene tubercle and adjoining ridge on inner border and upper surface of first (1st) rib between subclavian artery and subclavian vein. Thus it separates subclavian vein from subclavian artery and divides the subclavian artery (which lies posterior to it) in three (3) parts – 1st medial, 2nd posterior and 3rd part lateral to it. Phrenic nerve covered with a prevertebral layer of deep cervical fascia run vertically down on the anterior surface of scalenus anterior muscle (but do not pierce it). Ascending cervical artery (a branch of inferior thyroid artery or thyrocervical trunk) runs along with phrenic nerve but medial to it. Whereas transverse cervical and suprascapular arteries lie anterior to scalenus anterior and posterior to the lateral part of the carotid sheath (containing internal jugular vein), run transversely and bind phrenic nerve to muscle.