CHAPTER XV. 



THE ARM AND FORE-ARM. 



IN 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 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. 86, //), the axis of which is 

 directed upwards and backwards. 1 This, in the living 

 animal, is covered with a 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 bidpital 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 



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

 assumes in the ordinary attitude of a quadruped while standing or 

 walking. 



