132 



OSTEOLOGY. 



Fig. 88. Left Ilumerns. Posterior Surface. 



to the interval between the radius and 

 ulna. The internal border is thicker, 

 more prominent, and consequently of 

 greater length than the external. The 

 grooved portion of the articular surface fits 

 accurately within the greater sigmoid ca- 

 vity of the ulna ; it is broader and deeper 

 on the posterior than on the anterior aspect 

 of the bone, and is directed obliquely from 

 behind forwards, and from without in- 

 wards. Above the back part of the 

 trochlear surface, is a deep triangular de- 

 pression, the olecranon depression in which 

 is received the summit of the olecranon 

 process in extension of the forearm. Above 

 the front part of the trochlear surface, is 

 seen a small depression, the coronoid de- 

 pression; it receives the coronoid process 

 of the ulna during flexion of the fore- 

 arm. These fossa3 are separated from one 

 another by a thin transparent laminae of 

 bone, which is sometimes perforated; their 

 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. Above the front part of the radial 

 tuberosity, 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 pro- 

 minent than the internal, 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 supi- 

 nator muscles. The internal condyle, 

 larger and more prominent than the ex- 

 ternal, is directed a little backwards, and 

 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 di- 

 rectly continuous above with the external 

 and internal borders. The greater pro- 

 minence 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. 

 89): one for the shaft, one for the head, 



