HUMERUS. 



211 



Fig. 15R. Plan of the Development of the 

 Humerus. By Seven Centres. 



the radial tuberositj is seen a slight depression, which receives the anterior 

 border of the head of the radius when the forearm is strongly flexed. The 

 external condyle is a small tubercular eminence, less prominent than the inter- 

 nal, curved a little forwards, and giving attachment to the external lateral 

 ligament of the elbow-joint, and to a tendon common to the origin of some of 

 the extensor and supinator muscles. The internal condyle, larger and more 

 prominent than the external, is directed a little backwards ; it gives attachment 

 to the internal lateral ligament, and to a tendon common to the origin of some 

 of the flexor muscles of the forearm. These eminences are directly continuous 

 above with the external and internal borders. The greater prominence of the 

 inner one renders it more liable to fracture. 



Structure. The extremities consist of cancellous tissue, covered with a thin 

 compact layer; the shaft is composed of a cylinder of compact tissue, thicker 

 at the centre than at the extremities, and hollowed out by a large medullary 

 canal, which extends along its whole length. 



Development. By seven centres (Fig. 156) one for the shaft, one for the 

 head, one for the greater tuberosity, one for the radial, one for the trochlear 

 portion of the articular surface, and one for each condyle. The centre for the 

 shaft appears very early, soon after ossification has commenced in the clavicle, 

 and soon extends towards the extremities. At birth the humerus is ossified 

 nearly in its whole length, the extremities remaining cartilaginous. Between 

 the first and second years ossification commences in the head of the bone, and 

 between the second and third years the centre for the tuberosities makes its 

 appearance, usually by a single ossific point, but sometimes, according to 

 Beclard, by one for each tuberosity, that 

 for the lesser being small, and not appear- 

 ing until after the fourth year. By the 

 fifth year the centres for the head and 

 tuberosities have enlarged and become 

 joined, so as to form a single large epi- 

 physis. 



The lower end of the humerus is de- 

 veloped in the following manner: At 

 the end of the second year ossification 

 commences in the radial portion of the 

 articular surface, and from this point ex- 

 tends inwards, so as to form the chief part 

 of the articular end of the bone, the centre 

 for the inner part of the articular surface 

 not appearing until about the age of twelve. 

 Ossification commences in the internal 

 condyle about the fifth year, and in the 

 external one not until between the thir- 

 teenth or fourteenth year. About sixteen 

 or seventeen years, the outer condyle and 

 both portions of the articulating surface 

 (having already joined) unite with the 

 shaft; at eighteen years the inner condyle 

 becomes joined, whilst the upper epiphysis, 

 although the first formed, is not united 

 until about the twentieth year. 



Articulations. With the glenoid cavity 

 of the scapula, and with the ulna and 

 radius. 



Attachment of Muscles. To the greater 



tuberosity, the Supraspinatus, Infraspinatus, and Teres Minor; to the lesser 

 tuberosity, the Subscapularis; to the anterior bicipital ridge, the ^ectoralis 



Tu&erosities lie ml at I jy 

 5 . a? and unitr j 



"haft 



Unites wrtl 

 Shaft: at 



