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The proximal extremity is the thickest and stoutest part of the bone. The caput humeri (head), which forms about one-third of a spheroid and is covered with
articular edge of the groove opposite the lesser tubercle is usually notched for the attachment of the superior gleno-humeral ligament. The tuberculum majus (greater
tubercle) abuts on the lateral side of the head and becomes continuous with the body distally. Its proximal surface forms a quadrant, which is subdivided into three more or less smooth areas of unequal size. Of these the upper and anterior is for the insertion of the supraspinatus muscle, the middle for the infraspinatus, whilst the most distal and posterior serves for the insertion of the teres minor muscle. The lateral surface of this tubercle, which bulges beyond the line of the shaft, is rough and pierced by numerous vascular foramina. Anteriorly the greater tubercle is separated from the tuberculum minus (lesser tubercle) by a well-defined furrow, called the sulcus intertubercularis (intertubercular groove) (O.T. bicipital groove). The transverse humeral ligament stretches across the groove between the two tubercles, thus converting the groove into a canal in which the tendon of the long head of the biceps and the ascending articular branch of the anterior circumflex artery of the humerus are lodged. The lesser tubercle lies in front of the lateral half of the head; it forms a pronounced elevation, which fades into the shaft distally. The surface of this tubercle is faceted above and in front for the insertion of the subscapularis muscle, whilst laterally it forms the prominent medial lip of the inter-tubercular groove. Distal to the head and tubercles the shaft of the bone rapidly contracts, and is here named the collum chirurgicum (surgical neck) owing to its liability to fracture at this spot.
The corpus humeri (body, or shaft) is cylindrical in its proximal half. On it the inter-tubercular groove may be traced distally and slightly medially, along its anterior surface. The edges of the groove, which are termed its lips, are confluent proximally with the greater and lesser tubercles, respectively. Here they are prominent, and form the crista tuberculi majoris et minoris (crests of the greater and lesser tubercles). Distally the lips of the intertubercular groove gradually fade away, the medial more rapidly than the lateral, which latter may usually be traced distally to a rough elevation placed on the lateral anterior surface of the shaft about its middle, called the deltoid tuberosity. Into the lateral lip of the intertubercular groove are inserted the fibres of the pectoralis major tendon; hence it is sometimes described
FIG. 196.-POSTERIOR SURFACE OF THE RIGHT HUMERUS.
as the pectoral ridge. To the floor of the groove the latissimus dorsi is attached; whilst the teres major muscle is inserted into the medial lip.
The tuberositas deltoidea (deltoid tuberosity), to which the powerful deltoid muscle is attached, is a rough, slightly elevated V-shaped surface, placed on the lateral anterior surface of the body about its middle. The anterior limb of the V is parallel to the axis of the body, and is continuous proximally with the lateral lip of the intertubercular groove, whilst the posterior limb of the V winds obliquely round the lateral anterior surface of the bone towards the posterior surface, where it becomes continuous with a slightly elevated and occasionally rough ridge which leads proximally along the posterior aspect of the bone towards the greater tubercle; from this latter ridge the lateral head of the triceps muscle arises.
The medial anterior surface of the body about its middle inclines to form a rounded border, on which there is often a rough linear impression marking the insertion of the coracobrachialis muscle. Distal to this the body becomes compressed from before backwards and expanded from side to side, ending distally on each side in an epicondyle. Its surfaces are now anterior and posterior, being separated from each other by two clearly defined borders, the medial and lateral margins, or epicondylic ridges. Of these, the medial is the more curved and less prominent, and is continuous proximally with the surface to which the coracobrachialis is attached, whilst distally it ends by blending with the medial epicondyle. The lateral is straighter and more projecting; its edge is usually distinctly lipped. Confluent with the lateral epicondyle distally, it may be traced proximally to near the deltoid tuberosity, where it turns backwards more or less parallel to the posterior oblique border of that impression, to be lost on the posterior surface of the body. The interval between this border and the MEDIAL HEAD OF deltoid eminence is thus converted into a shallow oblique furrow, which winds round the lateral surface of the bone just distal to its middle; this constitutes the groove for the radial nerve along which the radial (O.T. musculo-spinal) nerve, together with the profunda brachii artery, passes from the back to reach the front of the arm. To the epicondylic ridges are attached the intermuscular septa, whilst the lateral in its proximal two-thirds furnishes a surface for the origin of the brachioradialis muscle, and in its distal third for the extensor carpi radialis longus muscle.
The anterior surface of the distal half of the body is of elongated triangular form, the base corresponding to the distal extremity of the bone. Running down the centre of this is a broad, rounded, elevated ridge, most pronounced proximally, where it joins the deltoid tuberosity, and sloping on either side towards the epicondylic ridges; it is into the lateral of these slopes that the groove for the radial nerve passes. Distally the elevated surface spreads out, and becomes confluent with the epicondyles. The epicondylus medialis (medial epicondyle) is the more prominent of the two, and furnishes a surface for the origin of the pronator teres, and the superficial flexor muscles of the forearm. The epicondylus lateralis (lateral epicondyle), stunted and but little projecting, serves for the attachment of the common tendon of origin of the extensor muscles. The brachialis muscle has an extensive origin from the anterior surface of the distal half of the body, including between its proximal slips the insertion of the deltoid.
FIG. 197.-POSTERIOR SURFACE OF THE
The posterior surface of the distal half of the body is smooth and rounded from
side to side; somewhat flattened distally, where the whole body tends to incline forwards, it becomes continuous on either side with the posterior surfaces of the epicondyles, the medial of which is grooved for the passage of the ulnar nerve, whilst the lateral supplies an origin for the anconæus muscle. The medial head of the triceps muscle has an extensive attachment from the posterior Surface of the distal two-thirds of
FIG. 198.-PROXIMAL ASPECT OF THE HEAD OF THE RIGHT HUMERUS (with the outline of the distal extremity in relation the body, medial to and distal to the groove for the radial nerve.
thereto shown in dotted line).
The distal extremity of the humerus is furnished with two articular surfaces (the condyles proper), the lateral of which, called the capitulum, for articulation with the proximal surface of the head of the radius, is a rounded eminence, placed on the anterior surface and distal border, but not extending on to the posterior surface. of the distal end of the bone. Proximal to it, in front, there is a shallow depression (fossa radialis), into which the margin of the head of the radius sinks when the elbow is strongly flexed. A shallow groove separates the capitulum medially from the trochlea, which is a grooved articular surface, with prominent edges winding spirally round the distal extremity of the body. The spiral curves from behind forwards and medially, and its axis is slightly oblique to the long axis of the shaft. The medial lip is the more salient of the two, and forms a sharp and well-defined margin to the articular area; its cartilage-covered surface is slightly convex. The lateral lip, much less prominent, is rounded off into the articular groove which separates it from the capitulum, posterior to which, however, it is carried up as a more or less definite crest. It is by means of the trochlea that the humerus articulates with the semilunar notch of the ulna. On the anterior surface of the bone, immediately proximal to the trochlea, is a depressionthe fossa coronoidea (coronoid fossa)-into which the coronoid process of the ulna slips in flexion of the joint, whilst in a corresponding position on the posterior aspect of the distal end of the body there is a hollow, called the fossa olecrani (olecranon fossa), just proximal to the trochlea posteriorly. Into this the olecranon process sinks when the elbow is extended. The two fosse are separated by a thin translucent layer of bone which may be deficient, thus leading to the formation of a
FIG. 200.-THE DISTAL END OF THE foramen between the two hollows in the macerated RIGHT HUMERUS SEEN FROM THE bone. The anterior part of the capsule of the elbowjoint is attached to the proximal margins of the radial and coronoid fossæ in front, whilst the posterior ligament is connected with the proximal border and lateral edges of the olecranon fossa behind. The strong
OLECRANON FOSSA GROOVE FOR ULNAR NERVE FIG. 199.-DISTAL ASPECT OF THE DISTAL EXTREMITY OF THE RIGHT HUMERUS.
ulnar and radial collateral ligaments of the elbow joint are attached proximally to the medial and lateral epicondyles respectively. The proportionate length of the humerus to the body height is as 1 is to 4.93-5.25.
Nutrient foramina are usually to be seen, one at or near the surface for the insertion of the coraco-brachialis, the other usually close to the posterior border of the deltoid tuberosity; both have a distal direction. Numerous vascular foramina are scattered along the line of the anatomical neck, the larger ones being situated near the proximal end of the inter-tubercular groove. The vascularity of the bone is here intimately associated with the activity of its growth in this situation. Connexions.-The humerus articulates proximally with the scapula, and distally with the radius and ulna. Embedded, as the humerus is, in the substance of the arm, its body and head are surrounded on all sides. It is only at its distal part that it comes into direct relation with the surface, the medial epicondyle forming a characteristic projection on the medial side of the elbow; whilst the lateral epicondyle, less prominent, and the lateral epicondylic ridge can best be recognised when the elbow is bent.
Sexual differences.-Dwight (American Journ. of Anat. vol. iv. 1904) has shown that the e.. head of the humerus in the female is proportionately smaller than that of the male.
Ossification. At birth the body of the humerus is usually the only part of the bone ossified, if we except the occasional presence (22 per cent.) of an ossific centre in the head. (H. R. Spencer, Journ. Anat. and Physiol. vol. xxv. p. 552.) The centre for the body makes its appearance early in the second month of intra-uterine life. Within the first six months after birth a centre usually appears for the head; this is succeeded by one for the greater tubercle during the second or third year. These soon coalesce; and a third centre for the lesser tubercle begins to appear about the end of the third year, or may be delayed till the fourth or fifth year. These three centres are all blended by the seventh year, and form an epiphysis, which ultimately unites with the body about the age of twenty-five. It may be noticed that the proximal end of the diaphysis is conical and pointed in the centre, over which the epiphysis fits as a cap, an arrangement which thus tends to prevent its displacement before union has occurred. The first centre to appear in the distal extremity is that for the capitulum about the second or third This extends medially, and forms the lateral half of the trochlear surface, the centre for the medial half not making its appearance till the eleventh or twelfth year.