Humerus vs. Radius vs. Ulna

Difference Between Humerus And Radius And Ulna

Humerus

In the upper arm, there is but one bone, the humerus. Its knoblike upper end forms the ball portion of the ball-and-socket shoulder joint; the scapula forms the socket portion. The four muscles that immediately surround the shoulder joint are attached to the outer part of the knob. Since the scapula and the humerus fit together rather poorly, these muscles must hold the bones closely together.

The upper part of the humerus shaft is at first cylindrical. As it approaches the elbow, it flattens out and expands sideways. At this expanded end are two joint surfaces that lie side by side. The inner surface resembles a spool, pulley, or hourglass laid on its side. It is known as the trochlea, the Greek word for “pulley.” The inner bone of the forearm, the ulna, hooks onto the trochlea from behind.

Radius and Ulna

The forearm contains two bones: the ulna and the radius. The ulna is primarily involved in the bending and straightening of the elbow—it is thick and massive at the elbow while tapering to a disklike head at the wrist. The upper end of the ulna looks like a monkey wrench. Its jaws grasp the lower end of the humerus. The part of the ulna that bends around the trochlea of the humerus is called the funny bone; this bone projects from the elbow. The nerve running to the ulna passes close by the funny bone, which is why, when this bone is bumped, it tingles.

The radius is primarily responsible for carrying the hand. As a result, it is thick and heavy at the wrist while tapering (to the elbow) in the same kind of disklike head that the ulna displays at the wrist.

 

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