CHAPTER XV. 



THE ARM AND FORE-ARM. 



the upper segment of the limb proper there is always 

 one bone, the Humerus ; in the second segment, two bones 

 placed side by side, the Radius and the Ulna. 



The Humerus (except in some of its extreme modifica- 

 tions) is more or less elongated and cylindrical. It is 

 described as having a shaft, and two extremities. The upper 

 or proximal extremity has a smooth, convex, generally more 

 or less rounded head (Fig. 81, h), the axis of which is 

 directed upwards and backwards. 1 This, in the living 

 animal, is covered with , sl thin layer of cartilage, and 

 articulates by a synovial joint with the glenoid cavity of the 

 shoulder girdle. The head is marked off from the shaft 

 very indistinctly by a constriction called the neck, imme- 

 diately below which, upon the anterior surface of the bone, 

 are two rough prominences (t and /') for the attachment of 

 muscles, called tuberosities, separated from each other by a 

 groove (bg) called the bicipital groove, as the tendon of the 

 biceps muscle runs in it after arising from the margin of the 

 glenoid fossa. The tuberosities are generally distinguished 



The terms of relative position here used are those which the bone 

 assumes in the ordinary attitude of a quadruped while standing or 

 walking. 





