Supraspinatus

The supraspinatus is a triangular muscle of the shoulder girdle. 

Origin: supraspinous fossa of the scapula. The muscle completely fills the supraspinous fossa and its fibers pass laterally.

Insertion: greater tubercle of the humerus. 

Action: this is one of the rotator cuff muscles providing abduction of the upper arm and stabilizing the shoulder.

Innervation: suprascapular nerve, which arises from the 5th and 6th cervical nerve roots of the brachial plexus.

Blood supply: suprascapular artery, a branch of the thyrocervical trunk.

Infraspinatus

The infraspinatus is a flat, triangular shaped muscle of the shoulder girdle. 

Origin: infraspinous fossa on the posterior surface of the scapula. This muscle completely fills the infraspinous fossa of the scapula and its fibers pass laterally. 

Insertion: greater tubercle of the humerus. 

Action: it is also one of the rotator cuff muscles that stabilizes the shoulder and provides external rotation of the upper arm. 

Innervation: suprascapular nerve, which arises from the 5th and 6th cervical nerve roots of the brachial plexus.

Blood supply: suprascapular and circumflex scapular arteries.