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The humerus is an os longum in the arm or fore-legs (animals) that runs from the shoulder to the elbow. On a skeleton, it fits between the scapula, and the membri antebrachii, radius and ulna.
, there is a ball-and-socket joint, where the head of the humerus caput humeri articulates with the cavitas glenoidalis of the scapula. This type of joint allows a wide range of movement.
The distal end of the humerus (at the elbow) creates a hinge joint with the ulna, allowing only flexion and extension. This happens on the trochlea of the humerus. Two pits at this end of the humerus (the coronoid fossa and the olecranon fossa) allow the ulna room to move, but prevent it from over-flexing/extending.
There is also a pivot joint between the capitulum of the humerus, and the head of the radius. This allows the hand to pronate and supinate (turn to face downwards or upwards).
A variety of muscles attach to the humerus. These enable movement at the elbow and at the shoulder.
The rotator cuff muscles attach at the proximal humerus, and can rotate and abduct the arm at the shoulder.
Deltoid has a variety of actions on the top of the arm. Pectoralis major, teres major and latissimus dorsi, which all insert at the intertubercular groove of the humerus, adduct and medially rotate the humerus.
Biceps brachii, brachialis, coracobrachialis, and brachioradialis (which attaches very distally), act to flex the elbow. Biceps however does not attach to the humerus.
Triceps brachii and anconeus extend the elbow, and attach to the posterior side of the humerus.
Some of the forearm muscles, (such as
The medial border extends from the lesser tubercle to the medial epicondyle. Its upper third consists of a prominent ridge, the crest of the lesser tubercle, which gives insertion to the tendon of the Teres major. About its center is a slight impression for the insertion of the Coracobrachialis, and just below this is the entrance of the nutrient canal, directed downward; sometimes there is a second nutrient canal at the commencement of the radial sulcus. The inferior third of this border is raised into a slight ridge, the medial supracondylar ridge, which becomes very prominent below; it presents an anterior lip for the origins of the Brachialis and Pronator teres, a posterior lip for the medial head of the Triceps brachii, and an intermediate ridge for the attachment of the medial intermuscular septum.
The antero-lateral surface is directed lateralward above, where it is smooth, rounded, and covered by the Deltoideus; forward and lateralward below, where it is slightly concave from above downward, and gives origin to part of the Brachialis. About the middle of this surface is a rough, triangular elevation, the deltoid tuberosity for the insertion of the Deltoideus; below this is the radial sulcus, directed obliquely from behind, forward, and downward, and transmitting the radial nerve and profunda artery.
The antero-medial surface, less extensive than the antero-lateral, is directed medialward above, forward and medialward below; its upper part is narrow, and forms the floor of the intertubercular groove which gives insertion to the tendon of the Latissimus dorsi; its middle part is slightly rough for the attachment of some of the fibers of the tendon of insertion of the Coracobrachialis; its lower part is smooth, concave from above downward, and gives origin to the Brachialis. The posterior surface appears somewhat twisted, so that its upper part is directed a little medialward, its lower part backward and a little lateralward. Nearly the whole of this surface is covered by the lateral and medial heads of the Triceps brachii, the former arising above, the latter below the radial sulcus.
The lower extremity is flattened from before backward, and curved slightly forward; it ends below in a broad, articular surface, which is divided into two parts by a slight ridge. Projecting on either side are the lateral and medial epicondyles. The articular surface extends a little lower than the epicondyles, and is curved slightly forward; its medial extremity occupies a lower level than the lateral. The lateral portion of this surface consists of a smooth, rounded eminence, named the capitulum of the humerus; it articulates with the cupshaped depression on the head of the radius, and is limited to the front and lower part of the bone. On the medial side of this eminence is a shallow groove, in which is received the medial margin of the head of the radius. Above the front part of the capitulum is a slight depression, the radial fossa, which receives the anterior border of the head of the radius, when the forearm is flexed. The medial portion of the articular surface is named the trochlea, and presents a deep depression between two well-marked borders; it is convex from before backward, concave from side to side, and occupies the anterior, lower, and posterior parts of the extremity. The lateral border separates it from the groove which articulates with the margin of the head of the radius. The medial border is thicker, of greater length, and consequently more prominent, than the lateral. The grooved portion of the articular surface fits accurately within the semilunar notch of the ulna; it is broader and deeper on the posterior than on the anterior aspect of the bone, and is inclined obliquely downward and forward toward the medial side. Above the front part of the trochlea is a small depression, the coronoid fossa, which receives the coronoid process of the ulna during flexion of the forearm. Above the back part of the trochlea is a deep triangular depression, the olecranon fossa, in which the summit of the olecranon is received in extension of the forearm. These fossæ are separated from one another by a thin, transparent lamina of bone, which is sometimes perforated by a supratrochlear foramen; they are lined in the fresh state by the synovial membrane of the elbow-joint, and their margins afford attachment to the anterior and posterior ligaments of this articulation. The lateral epicondyle is a small, tuberculated eminence, curved a little forward, and giving attachment to the radial collateral ligament of the elbow-joint, and to a tendon common to the origin of the Supinator and some of the Extensor muscles. The medial epicondyle, larger and more prominent than the lateral, is directed a little backward; it gives attachment to the ulnar collateral ligament of the elbow-joint, to the Pronator teres, and to a common tendon of origin of some of the Flexor muscles of the forearm; the ulnar nerve runs in a groove on the back of this epicondyle. The epicondyles are continuous above with the supracondylar ridges.
The extremities consist of cancellous tissue, covered with a thin, compact layer [Fig. 3]; the body is composed of a cylinder of compact tissue, thicker at the center than toward the extremities, and contains a large medullary canal which extends along its whole length.
(Figs. 210, 211) The humerus is ossified from eight centers, one for each of the following parts: the body, the head, the greater tubercle, the lesser tubercle, the capitulum, the trochlea, and one for each epicondyle. The center for the body appears near the middle of the bone in the eighth week of fetal life, and soon extends toward the extremities. At birth the humerus is ossified in nearly its whole length, only the extremities remaining cartilaginous. During the first year, sometimes before birth, ossification commences in the head of the bone, and during the third year the center for the greater tubercle, and during the fifth that for the lesser tubercle, make their appearance. By the sixth year the centers for the head and tubercles have joined, so as to form a single large epiphysis, which fuses with the body about the twentieth year. The lower end of the humerus is ossified as follows. At the end of the second year ossification begins in the capitulum, and extends medialward, to form the chief part of the articular end of the bone; the center for the medial part of the trochlea appears about the age of twelve. Ossification begins in the medial epicondyle about the fifth year, and in the lateral about the thirteenth or fourteenth year. About the sixteenth or seventeenth year, the lateral epicondyle and both portions of the articulating surface, having already joined, unite with the body, and at the eighteenth year the medial epicondyle becomes joined to it.
This article is based on an entry from the 1918 edition of Gray's Anatomy, which is in the public domain. As such, some of the information contained herein may be outdated. Please edit the article if this is the case, and feel free to remove this notice when it is no longer relevant.
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