THE HUMERUS 151 



cavity on the summit of the radius. The radial fossa is a sUght depression on the 

 front of the bone, immediately above the capitulum, which receives the anterior 

 edge of the head of the radius in complete flexion of the forearm, whilst between 

 the capitulum and the trochlea is a shallow groove occupied by the medial margin 

 of the head of the radius. 



In the recent state the inferior articular surface is covered with cartilage, the fossae are lined 

 by synovial membrane, and their margins give attachment to the capsule of the elbow-joint. 

 Projecting on either side from the lower end of the humerus are the two epicondyles. The 

 medial one is large and by far the more prominent of the two, rough in front and below, smooth 

 behind, where there is a shallow groove for the ulnar nerve. The rough area serves for origin 

 of the pronator teres above, the common tendon of origin of the^xor carpi radialis, palmaris 

 longus, flexor digitorum sublimis and flexor carpi ulnaris in the middle, and the ulnar collateral 

 hgament below. The lateral epicondyle is flat and irregular. Above, it gives attachment to a 

 common tendon of origin of the extensor carpi radialis brevis, extensor digitorum communis, 

 extensor quinti digiti proprius, extensor carpi ulnaris, and supinator; to a depression near the outer 

 margin of the capitulum, the radial collateral ligament is attached, and from an area below and 

 behind, the anconeus takes origin. 



Architecture. — The interior of the shaft of the humerus is hollowed out by a large medullary 

 canal, whereas the extremities are composed of cancellated tissue invested by a thin compact 

 layer. The arrangement of the cancellous tissue at the upper end of the humerus is shown 

 in fig. 177. The lamellae converge to the axis of the bone and form a series of superimposed 

 arches which reach upward as far as the epiphysial line. In the epiphyses the spongy tissue 

 forms a fine network, the lamellae resulting from "pressure" being directed at right angles to 

 the articular surface of the head and to the great tuberosity. 



Blood-supply. — The foramina which cluster round the circumference of the head and tuber- 

 osities transmit branches from the transverse scapular (suprascapular) and anterior and pos- 

 terior circumflex arteries. At the top of the intertubercular groove is a large nutrient foramen 



Fig. 179. — The Head of the Humerus at the Sixth Year. (In section.) 



The centre for the head appears 

 during the first year; it is some- 

 times present at birth 



The centre for the greater tuberosity 

 appears in the third year 



for a branch of the anterior circumflex artery which suppUes"jthe head. The nutrient artery of 

 the shaft is derived from the brachial, and in many cases, an additional branch, derived from the 

 profunda artery, enters the foramen in the groove for the radial nerve (musculo-spiral groove) . 

 The lower extremity is nourished by branches derived from the profunda (superior profunda) , 

 the superior and inferior ulnar collateral (inferior profunda and anastomotic), and the recurrent 

 branches of the radial, ulnar, and interosseous arteries. 



Ossification. — The humerus is ossified from one primary centre (diaphysial) and slx second- 

 ary centres (epiphysial). The centre for the shaft appears about the eighth week of intra- 

 uterine fife and grows very rapidly. At birth only the two extremities are cartilaginous, and 

 these ossify in the following manner: Single centres appear for the head in the first year, for 

 the greater tuberosity in the third year, and for the lesser tuberosity in the fifth year, though 

 sometimes the latter ossifies by an extension from the greater tuberosity. These three nuclei 

 coalesce at six years to form a single epiphysis, which joins the shaft about the twentieth year. 



The inferior extremity ossifies from four ceatres: one for the capitulum appears m the third 

 year, a second for the medial epicondyle in the fifth year, a third for the trochlea in the tenth 

 year, and a fourth for the lateral epicondyle in the fourteenth year. The nuclei for the capitu- 

 lum, trochlea, and lateral epicondyle coalesce to form a single epiphysis which joins the shaft 

 in the seventeenth year. The nucleus of the medial epicondyle joins the shaft independently 

 at the age of eighteen years. . . 



A study of the upper end of the humeral shaft before its union with the epiphysis is ot interest 

 in relation to what is known as the neck of the humerus. The term neck is apphed to three 

 parts of this bone. The anatomical neck is the constriction to which the articular capsule is 

 mainly attached, and its position is accurately indicated by the groove which hes internal to 

 the tuberosities. The upper extremity of the humeral shaft, before its union with the epiphysis , 

 terminates in a low three-sided pyramid, the surfaces of which are separated from one another 

 by ridges. The medial of these three surfaces underhes the head of the bone, and the two 

 lateral surfaces underhe the tuberosities. The part supporting the head constitutes the morpho- 

 logical neck of the humerus, whilst the surgical neck is the indefinite area below the tuberosities 

 where the bone is liable to fracture. 



