The humerus is a long cylindrical bone. The superior head is round and covered with cartilage; just below it is the anatomical neck ; further down, below the tuberosities, is the surgical neck. There are two tuberosities, the greater on the outside and the lesser on the inside ; between them is the bicipital groove. This groove is lined with cartilage, and receives the tendon of the long head of the biceps (a muscle

on the front of the arm). The ridges at each side form points of attachment for muscles of the trunk. The shaft has an oblique depression going down the outer aspect, called the musculo-spiral groove, through which the musculo-spiral nerve runs.

The bone is somewhat twisted. The lower end is flattened, and has two projections called condyles, an inner and an outer. At the back, between the two condyles, is a fossa, or depression, into which the olecranon process of the ulna fits when the forearm is extended ; in front there is a smaller fossa, into which the coronoid process of the ulna fits when the forearm is flexed.

An articulating surface on the inner side for the ulna is called the trochlear surface, and that for the radius on the outer side is called the capitellum. They are separated by a narrow groove. Most of the muscles of the forearm and hand arise from the condyles of the humerus.

Fig. 32.—Humerus (Left).

1, Head ; 2, great tuberosity ; 3, lesser tuberosity ; 4, bicipital groove ; 5, coronoid fossa ; 6, external condyle ; 7, internal condyle ; 8, troch lea; 9, capitellum.