The arm - or upper arm - is formed by a single bone surrounded by muscles, which, with the exception of the biceps, are attached to it. The main vessels and most of the important nerves run down its inner side. It receives from the trunk the insertions of the muscles which move it, and gives origin to the muscles which move the forearm. It is more subject to injury than to disease; infection, caries, and rickets may attack the bone and rarely new growths may occur, but its common affections are wounds involving the muscles, blood-vessels, or nerves, and fractures of the bone. Severe injuries occasionally necessitate amputation.

Fig. 277.   Anterior surface of humerus, showing attachment of muscles.

Fig. 277. - Anterior surface of humerus, showing attachment of muscles.

Fig. 278.   Posterior surface of humerus, showing attachment of muscles.

Fig. 278. - Posterior surface of humerus, showing attachment of muscles.