148 



THE SKELETON. 



Epiphyses of head and 

 tuberosities blend at 

 5th year, and unite 

 with shaft at 20th 

 year. 



Unites with shaft ) 

 at 18th year. J 



the forearm. These fossae are separated from one another by a thin, transparent 

 lamina of bone, which is sometimes perforated, forming the supratrochlear foramen ; 



their upper margins afford attachment to the 

 anterior and posterior ligaments of the elbow- 

 joint, and they are lined, in the recent state, 

 by the synovial membrane of this articula- 

 tion. The articular surfaces, in the recent 

 state, are covered with a thin layer of car- 

 tilage. The external condyle (cpicondyle] is 

 a small, tubercular ^tninence, less prominent 

 than the internal, curved a little forward, and 

 giving attachment to the external lateral lig- 

 ament of the elbow-joint, and to a tendon 

 common to the origin of some of the ex- 

 tensor and supinator muscles. The internal 

 condyle (epitrochlea), larger and more prom- 

 inent, and therefore more liable to fracture, 

 than the external, is directed a little backward : 

 it gives attachment to the internal lateral 

 ligament, to the Pronator radii teres, and to 

 a tendon common to the origin of some of 

 the flexor muscles of the forearm. The 

 ulnar nerve runs in a groove at the back 

 of the internal condyle, or between it and 

 the olecranon process. These condyles are 

 directly continuous above with the external 

 and internal supracondylar ridges. 



Structure. The extremities consist of 

 cancellous tissue, covered with a thin com- 

 pact layer; the shaft is composed of a cylin- 

 der of compact tissue, thicker at the centre 

 than at the extremities, and hollowed out 

 by a large medullary canal, which extends along its Avhole length. 



Development. By seven, or occasionally eight, centres (Fig. 99), one for the 

 shaft, one for the head, one for the tuberosities, one for the radial head, one for 

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

 nucleus for the shaft appears near the centre of the bone in the eighth week, and 

 soon extends toward the extremities. At birth the humerus is ossified nearly in 

 its whole length, 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 centre for the tuberosities makes its appearance, usually by a 

 single ossific point, but sometimes, according to Be'clard, by one for each tuber- 

 osity, that for the lesser being small and not appearing until the fifth year. By the 

 sixth year the centres for the head and tuberosities have increased in size and 

 become joined, so as to form a single large epiphysis. 



The lower end of the humerus is developed in the following manner : At the 

 end of the second year ossification commences in the capitellum, and from this 

 point extends inward, so as to form the chief part of the articular end of the bone, 

 the centre for the inner part of the trochlea 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 about the thirteenth or fourteenth year. About six- 

 teen 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; while 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. 



FIG. 99. Plan of the development of the 

 humerus, By seven centres. 



