General features.—The humerus is the longest and largest bone of the upper limb. It consists of expanded upper and lower extremities and a more or less cylindrical shaft. The rounded head occupies the upper and medial part of the upper end of the bone. The lesser tuberosity projects from the front of the shaft, close to the head, and is limited on its lateral side by a well-marked groove. By examination of the head and the lesser tuberosity the student should be able to assign a given humerus to its correct side.
The upper end of the humerus consists of the head, and the greater and lesser tuberosities (tubercles).
The head of the humerus (figs. 407, 411) forms rather less than half a sphere, and its smooth surface is covered with hyaline articular cartilage in life and in the unmacerated specimen. When the arm is at rest by the side, it is directed medially, backwards and upwards to articulate with the glenoid cavity of the scapula. The humeral articular surface is much more extensive than the glenoid cavity, and only a portion of it is in contact with the cavity in any one position of the arm. The margin of the head is most distinct in its medial and lower part.
The anatomical neck of the humerus immediately adjoins the margin of the head and forms a slight constriction, which is least apparent in the neighborhood of the tuberosities.
The lesser tuberosity (tubercle) is placed on the anterior aspect of the bone immediately beyond the anatomical neck, and shows a smooth, muscular impression on its upper part. Although thickly covered by muscle it can be felt through the skin. The lateral edge of the lesser tuberosity is sharp and forms the medial border of the bicipital groove.
The greater tuberosity occupies the lateral part of the upper end of the humerus and is the most lateral bony point in the shoulder region. It projects beyond the lateral border of the acromion and, covered by the thick, fleshy deltoid muscle, is responsible for the normal rounded contour of the shoulder. The portion of the tuberosity which adjoins the anatomical neck shows three flattened impressions for muscular attachments.
The bicipital groove (intertubercular sulcus) separates the two tuberosities and lodges the tendon of the long head of the biceps muscle.
The shaft of the humerus is almost cylindrical in its upper half but is triangular on section in its lower half, which is somewhat compressed in an anteroposterior direction. It presents three surfaces and three borders, which are not everywhere equally obvious.
The anterior harder commences above on the front of the greater tuberosity and runs downwards almost to the lower end of the bone. Its upper third forms the lateral lip of the bicipital groove and is roughened for muscular attachments. The succeeding portion is also roughened and forms the anterior limit of the deltoid tuberosity, but the lower half of the border is smooth and rounded.
The lateral border is conspicuous at the lower end of the bone, where its sharp edge is roughened along its anterior aspect. In the middle and upper thirds of the bone the border is barely discernible to the inexperienced eye, but in a well-marked bone it can be traced upwards to the posterior aspect of the greater tuberosity. About its middle the border is definitely interrupted by a wide, shallow groove which crosses the bone obliquely, passing downwards and forwards from its posterior to its anterior aspect. It is termed the spiral groove.
The medial border, although rounded, can be identified without difficulty in the lower half of the shaft. A little below the middle of the bone it presents a roughened strip, and above it becomes indistinct until it reappears as the medial lip of the bicipital groove. In this situation the border is again roughened and can be traced into the lesser tuberosity.
The anterolateral surface of the humerus lies between the anterior and the lateral borders. A little above its middle it is marked by a V-shaped roughened area, which is termed the deltoid tuberosity. The limbs of the V are broad and its apex is directed downwards. Behind the posterior limb of the V the spiral groove runs downwards and fades away on the lower part of the surface.
The anteromedial surface lies between the anterior and the medial borders of the bone. Rather less than its upper third forms the rough floor of the bicipital groove, but the rest of the surface is smooth. A little below its middle the nutrient foramen, which is directed downwards, opens close to the medial border.
The posterior surface lies between the medial and the lateral borders and is the most extensive surface of the three. Its upper third is crossed by a faint ridge, sometimes roughened, which runs obliquely downwards and laterally. The middle third is crossed by the commencement of the spiral groove. Rather more than the lower third forms an extensive, flattened surface, which widens considerably below. The lower end of the humerus (figs. 407, 412) is expanded transversely, and presents articular and non-articular portions.
The capitulum forms a rounded convex projection, considerably less than half a sphere, which covers the anterior and inferior aspects of the lower end of the humerus but does not extend on to its posterior aspect. It articulates with the disc like head of the radius, which lies in contact with its inferior aspect in full extension of the elbow but moves on to its anterior aspect when the joint is flexed.
The trochlea forms a pulley-shaped surface, which covers the anterior, inferior and posterior aspects of the lower end of the humerus. It is concave from side to side and convex from before backwards. On its lateral side it is separated from the capitulum by a faint groove, but its medial margin is salient and projects downwards beyond the rest of the bone. The trochlea articulates with the trochlear (semilunar) notch of the ulna. When the elbow is extended the inferior and posterior aspects of the trochlea are in contact with the ulna, but, as the joint is flexed, the trochlear notch rolls forwards on to the anterior aspect and the posterior aspect is then left uncovered. The downward projection of the medial edge of the trochlea is the principal factor in determining the angulation which is present between the long axis of the humerus and the long axis of the supinated forearm when the elbow is extended. The angle, which is of approximately 170�, is open to the lateral side and is termed the ‘carrying angle.� Owing to the existence of this angulation, the ulnar or medial border of the supinated and extended forearm cannot be brought into contact with the lateral aspect of the thigh, when the arm is by the side.
The non-articular part of the lower end of the humerus includes the medial and lateral epicondyles and the olecranon, coronoid and radial fossa.
The medial epicondyle forms a conspicuous blunt projection on the medial side of the lower end of the humerus. It is subcutaneous and can easily be identified through the skin. Its posterior aspect is smooth and is crossed by the large ulnar nerve, as it runs down into the forearm. In this situation the nerve can be felt and rolled against the bone. If the pressure exerted is sufficient, sensations are aroused identical with those produced by a knock on the `funny bone,’ when the nerve is jarred against the epicondyle. The lower part of the anterior and medial aspects of the medial epicondyle show an impression which gives attachment to the superficial group of the flexor muscles of the forearm.
The lateral epicondyle occupies the lateral part of the non-articular portion of the lower end of the humerus ; but does not project laterally beyond the lateral border. Its lateral and anterior aspects show a well-marked impression, which gives origin to the superficial group of the extensor muscles of the forearm. Its posterior aspect, which is very slightly convex, can easily be felt through the skin at the back of the elbow. The lateral border of the humerus terminates at the lateral epicondyle, and its lower portion is usually termed the lateral supracondylar ridge. The medial border of the humerus terminates below at the medial epicondyle, and its lower portion is usually termed the medial supracondylar ridge.
A deep hollow is situated on the posterior aspect of the lower end of the humerus, immediately above the trochlea. It is termed the olecranon fossa, on account of the fact that it lodges the tip of the olecranon of the ulna when the elbow is extended. The floor of the fossa is always thin and may be partially deficient. A similar but smaller hollow lies immediately above the trochlea on the anterior aspect of the lower end of the humerus and is termed the coronoid fossa. It provides room for the anterior margin of the coronoid process of the ulna during flexion of the elbow. A very slight depression lies above the capitulum on the lateral side of the coronoid fossa. It is termed the radial fossa, since it is related to the margin of the head of the radius in frill flexion of the elbow.
In lower mammals the long axes of the upper and lower articular surfaces of the humerus make an angle with each other of little more than 90�. In man, however, the upper end of the humerus appears to have been rotated laterally, so that the angle between the two axes has been increased to about 164�. This angulation is referred to as the angle of ‘humeral torsion.’ It is greater in men than women, in adults than children, and in higher races than in lower races and anthropoids. The significance of the torsion of the humerus has not yet been explained in a satisfactory planner.
The anatomical neck gives attachment to the capsular ligament of the shoulder-joint (figs- 408. 410), except at the upper end of the bicipital groove, where a deficiency exists to the long tendon of the biceps to emerge from the joint. On the medial side, however, the attachment extends downwards for 1 cm. or more on to the shaft of the bone.
The lesser tuberosity gives insertion to the subscapularis muscle (fig. 408) and its sharp lateral margin gives attachment to the medial end of the transverse ligament of the shoulder joint.
The greater tuberosity presents three muscular impressions (fig. 411). The uppermost receives the insertion of supraspinatus ; the middle impression gives insertion to the infraspinatus ; while the lowermost, which is placed on the posterior aspect of the tuberosity, gives insertion to the teres minor muscle (fig. 410). The projecting lateral aspect of the tuberosity presents numerous vascular foramina, and is covered by the deltoid muscle.
A part of the subacromial bursa may cover the upper part of this area and separate it from the muscle.
The oblique ridge which crosses the upper part of the posterior surface gives origin to the lateral head of the triceps. Above this muscle the circumflex (axillary) nerve and the posterior circumflex humeral vessels wind round this aspect of the bone under cover of the deltoid muscle. Below and medial to the origin of the lateral bead of the triceps, the spiral groove, containing the radial nerve and the profunda vessels, runs downwards and laterally to gain the anterolateral surface of the shaft. The area for the origin of the fleshy medial head of the triceps covers a very large part of the posterior surface of the bone. It forms an elongated triangular area, the apex of which lies on the medial part of the posterior surface above the level of the lower limit of the insertion of teres major. The area widens below and covers the whole surface almost down to the lower end of the bone.
The anterolateral surface of the humerus is smooth and featureless in its upper part, which is covered by the deltoid muscle. About, or a little above, the middle of this surface the deltoid muscle is inserted into the deltoid tuberosity, and below that level the surface gives origin to the lateral fibers of the brachialis, which extend upwards into the floor of the lower end of the spiral groove (fig. 410). The roughened anterior aspect of the lateral supracondylar ridge gives origin by its upper two-thirds to the brachioradialis and by its lower third to the extensor carpi radialis longus. Behind these muscles the ridge gives attachment to the lateral intermuscular septum of the arm.
The capsular ligament of the elbow-joint extends for some distance beyond the articular surface for its attachments to the humerus. Anteriorly it passes to the upper limits of the radial and coronoid fossa;, so that both these bony depressions are intracapsular and therefore lined with synovial membrane. Medially it is attached to the medial nonarticular aspect of the projecting lip of the trochlea and to the root of the medial epicondyle. Posteriorly it ascends to, or almost to, the upper margin of the olecranon fossa, which is therefore intracapsular and covered with synovial membrane. Laterally it skirts the lateral borders of the trochlea and capitulum, lying medial to the lateral epicondyle.
The muscular impression on the medial epicondyle gives attachment to the common origin of the superficial group of flexor muscles. They arise from the epiphysis for the epicondyle, but are entirely outside the articular capsule of the elbow-joint. The impression on the lateral epicondyle gives attachment to the common origin of the superficial group of extensor muscles of the. forearm. These arise from the lateral side of the lower numeral epiphysis, and, like the flexors, are situated outside the articular capsule. A small area on the posterior aspect of the lateral epicondyle gives origin to the anconeus. The medial epicondyle is curved backwards a little at its extremity, whereas the lateral epicondyle shows a slight curvature in the opposite direction.
A hook-shaped process of bone, termed the supracondylar process, varying from 2mm. to 20 mm, in length, is not infrequently found projecting from the anteromedial surface of the shaft of the humerus, about 5 cm. above the medial epicondyle. It is curved downwards and forwards, and its pointed end is connected to the medial border, just above the epicondyle, by a fibrous band which gives origin to a portion of the pronator teres ; through the foramen completed by this fibrous band the median nerve and brachial artery pass, when these structures deviate from their usual course. Sometimes the nerve alone is transmitted through it, or the nerve may be accompanied by the ulnar artery, in cases of high division of the brachial artery. A groove is usually found behind the process, in which the nerve and artery are lodged. This foramen is the homologue of the supracondylar foramen found in many animals, and probably serves in them to protect the nerve and artery from compression during the contraction of the muscles in this region.
Structure. -The ends of the humerus consist of spongy substance, covered with a thin layer of compact bone (fig. 413) ; the body is composed of a cylinder of compact bone, thicker at the center than towards the extremities, and a large medullary canal extends throughout its length.
Ossification (figs. 408, 414).-The humerus is ossified from eight centers, one for each of the following parts: the shaft, the head, the greater tuberosity, the lesser tuberosity, the capitulum and lateral part of the trochlea, the medial part of the trochlea, and one for each epicondyle. The center for the shaft appears near the middle in the eighth week of fetal life, and gradually extends towards the ends, which at birth are cartilaginous. During the first year, occasionally before birth, ossification begins in the head, during the third year in the greater tuberosity, and during the fifth in the lesser tuberosity. By the sixth year the centers for the head and tuberosities have joined to form a single large epiphysis, which is hollowed out on its inferior surface (fig. 413) to adapt it to the somewhat conical upper end of the diaphysis. It fuses with the shaft of the humerus about the twentieth year. The lower end its ossified as follows. At the end of the second year ossification begins in the capitulum and extends medially to form the chief park of the articular surface; the center for the medial part of the trochlea appears about the twelfth year. Ossification begins in the medial epicondyle about the fifth year, and in the lateral about the thirteenth or fourteenth year. The center for the lateral epicondyle fuses with those for the trochlea and capitulum (fig. 414), and the epiphysis thus formed unites with the shaft about the sixteenth or seventeeth year; the epiphysis for the medial epicondyle, which is entirely extracapsular (fig. 414), unites with the shaft about the eighteenth year.
Fractures of the humerus are common. This bone is probably in ore frequently fractured by muscular action than any other long bone; it is usually the shaft of the bone, just below the insertion of the deltoid, which is thus broken, and the accident has been known to happen from throwing a stone or a hand-grenade. Fracture of the surgical neck of the bone is not uncommon : impaction may occur, or the upper end of the lower fragment may be displaced into the axilla and damage the vessels or nerves. Separation of the upper epiphysis sometimes occurs in the young subject, and is marked by a characteristic deformity, the upper end of the diaphysis projecting abruptly at the front of The joint a short distance below the coracoid process. In fractures of the upper end of the humerus, extension with the arm in the abducted position is necessary- so that should ankylosis take place the mobility of the scapula may be brought into full use. In fractures of the body of the humerus the lesion may take place at any point, but appears to be more common in the lower than the upper part of the bone. The points of interest in connection with these fractures are : (1) that the radial nerve may be injured as it lies in the spiral groove, or may become involved in the callus which is subsequently- thrown out ; and (2) the frequency of non-union, which is believed to be more common in the humerus than in any other bone. The non-union is in some measure due to the difficulty in fixing the bone, since the upper end articulates with the movable scapula, and the shaft lies by the chest-wall, which moves with each respiration. Again, muscle is attached to the entire circumference of the hone and is liable to get between the fragments, should there be any overlapping of the latter. The circumflex (axillary) nerve may be injured by fractures of the upper end of the bone and the ulnar nerve by fracture of the medial epicondyle. In fractures of the lower end it is important to distinguish between those that involve the elbow-joint and those that do not ; the former are always serious, as they may lead to impairment of the utility of the limb ; they include the T-shaped fracture and oblique fractures which involve the articular surface. Those which do not involve the joint are the transverse fracture above the epicondyles, and the so-called epitrochlear fracture, where the tip of the medial epicondyle is broken off, generally by direct violence.
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