HUMERUS, THE UPPER ARM-BONE 



577 



HUMERUS, THE UPPER ARM-BON 



Lies between the scapula and elbow, in an oblique direction from the 

 point of the shoulder downwai"ds and backwards. 



Like all the long bones, the humerus may be described as consisting 

 of a body or shaft and two extremities. 



The body, which has a prismatic section, looks as if it had been twisted 

 on itself. In the upper pai^t it is expanded laterally to form the extei-nal 

 tuberosity (r,, Fig. 24) on the outside and the scabrous tubercle (4, Fig. 25) 

 internally, both being for the attachment of muscles. Towards the lower 

 extremity it becomes rounded, and then suddenly spreads out to present the 



4' 



Fig. 24.— Extkrnal Vp . 



1. Body of humerus with mnsculo-spiral groove. 



2. Head. 



3. External tubercle. 



4. Tubercles in front of head. 



5. External tuberosity. 



6. Sigmoid fossa. 



7. Olecranon fossa. 



8. External condyle. 



9. Internal condyle. 



wide surface which ends in the condyles on each side with the intervening 

 articular surfaces. 



The superior extremity, larger than the inferior, presents for consideration 

 a head and four tubercles. The head is the hemispherical smooth part, pro- 

 jecting posteriorly. It articulates with the glenoid cavity of the scapula, 

 which it much exceeds in extent of superficies. Anteriorly, the head is 

 surmounted by four tubercles ; three directly in front are between two 

 smooth grooves, which are covered with cartilage. The outer, or fourth 

 tubercle, is joined with the tuberosities by a protuberant ridge, the external 

 edge of which serves to guard against dislocation. 



The inferior extremity is made up of two condyles, or knuckle-like 

 projections (g, c,, Pig- 24), having between them the smooth articular sui'face for 

 the elbow joint. This surface is broad, and assumes a segmental form, being 



