The brachium refers to the upper arm, or the portion of the human arm between the shoulder and the elbow. This term is distinguished from the antebrachium, or forearm, which includes the parts of the arm between the elbow and the wrist. Also a zoological term, it can also reference the portion of an animal's limb that corresponds to the same region on the human skeleton.
Made up of a single long bone called the humerus, the upper arm contains one of the longest bones in the body. The top end of the humerus consists of a single ball joint, which sits inside the scapula, or shoulder joint. When seen from the front, the humerus appears to have the joint at an angle at the back of the bone; this is because the ball joint fits into the shoulder in an oblique manner called retroversion. At the other end of the humerus are two joints facing in nearly the opposite direction as the top joint. These two bottom joints mark the attachment of the antebrachium at the elbow.
Two major muscles are found in the brachium. The bicep houses a tendon that is attached through a furrow in the upper humerus, and the muscle itself sits on the front of the brachium. On the back side is the triceps, which attaches opposite the biceps. Tension in these muscles and their connective tissues flexes the arm, and the bumps at the top of the humerus not only provide a place for the tendon but increase leverage for the brachium.
A strong bone in adults, the brachium is partially made of cartilage in childhood. Bone replaces cartilage slowly and at a regular rate, so regular that examination of the cartilage-to-bone content can help determine the age of a child. A simple x-ray is needed to determine what state of development the arm is in. The first piece to change is the capitellum joint at the elbow, which turns to bone at about age two. The last is the lateral condyle on the outside of the base of the brachium, which turns to bone at about age 12.
Injuries to the brachium can affect other parts of the body and, in turn, the arm can be affected by injuries to other areas. A brachial plexus injury is sustained when there is damage to the nerves that run to the arm, and most commonly characterized by numbness and weakness in the brachium or loss of movement in any part of the arm or hand. The grooves at the bottom of the humerus also house nerve endings, including the spot known as the funny bone. Sudden pressure can create an electric pain, and chronic pressure on the base of the brachium can weaken the muscles of the hands.