The first rib is the most curved and usually the shortest of all the ribs; it is broad and flat, its surfaces looking upward and downward, and its borders inward and outward.

The head is small, rounded, and possesses only a single articular facet, for articulation with the body of the first thoracic vertebra.

The neck is narrow and rounded.

The tubercle, thick and prominent, is placed on the outer border. It bears a small facet for articulation with the Transverse Process of T1.

There is no angle, but at the tubercle the rib is slightly bent, with the convexity upward, so that the head of the bone is directed downward.

The upper surface of the body is marked by two shallow grooves, separated from each other by a slight ridge prolonged internally into a tubercle, the scalene tubercle

Scalene tubercle

The scalene tubercle is a small projection that runs along the medial border of the first rib between two grooves, which travel anteriorly for the subclavian vein and posteriorly for the subclavian artery. It projects outward medially and is the site of insertion for scalenus anterior....

, for the attachment of the Scalenus anterior

Scalenus anterior

The Scalenus anterior , also known as anterior scalene muscle, lies deeply at the side of the neck, behind the Sternocleidomastoideus.-Anatomy:...

; the anterior groove transmits the subclavian vein

Subclavian vein

The subclavian veins are two large veins, one on either side of the body. Their diameter is approximately that of the smallest finger.-Path:Each subclavian vein is a continuation of the axillary vein and runs from the outer border of the first rib to the medial border of anterior scalene muscle...

, the posterior the subclavian artery

Subclavian artery

In human anatomy, the subclavian arteries are two major arteries of the upper thorax , below the clavicle . They receive blood from the top of the aorta...

 and the lowest trunk of the brachial plexus

Brachial plexus

The brachial plexus is a network of nerve fibers, running from the spine, formed by the ventral rami of the lower four cervical and first thoracic nerve roots...

. Behind the posterior groove is a rough area for the attachment of the Scalenus medius

Scalenus medius

The Scalenus medius, the largest and longest of the three scalene muscles, arises from the posterior tubercles of the transverse processes of the lower six cervical vertebræ. It descendes along the side of the vertebral column to insert by a broad attachment into the upper surface of the first...

.

The under surface is smooth, and destitute of a costal groove.

The outer border is convex, thick, and rounded, and at its posterior part gives attachment to the first digitation of the Serratus anterior.

The inner border is concave, thin, and sharp, and marked about its center by the scalene tubercle.

The anterior extremity is larger and thicker than that of any of the other ribs.