THE APPENDICULAR SKELETON 51 



ii. The mesostemum is a rod of cartilage, ensheathed 

 in bone, projecting backwards behind the cora- 

 coids. 



iii. The metasteruum or xiphisternum is a broad ex- 

 panded plate of cartilage at the hinder end of 

 the sternum. 



2. The Fore-limb. 



The bones of the fore-limb are all cartilage-bones. With the 

 exception of the small bones of the wrist, they are elongated, 

 with enlarged ends capped with ai-ticular cartilage. The 

 enlarged ends or epiphyses ossify independently of the shaft of 

 the bone, with which they do not unite until late in life. The 

 end of a bone which, when the limb is extended, is nearer to 

 the body, is called its proximal end, the opposite extremity the 

 distal end. 



a. The arm. In the arm there is only a single bone. 



i. The humerus. The proximal end or head is en- 

 larged, and articulates with the glenoid cavity 

 of the pectoral girdle : below the head is the 

 strong deltoid ridge extending along the proximal 

 half of"tKFanterior surface. At the distal end 

 is a spheroidal articular surface for the bone of 

 the forearm : and at either side of this a 

 prominent condylar ridge, the inner or postaxial 

 one being the larger of the two, especially in the 

 male frog. 



b. The forearm. 



i. The radio-ulna corresponds to two bones, radius 

 and ulna, in other animals : it is single at its 

 proximal end, but in its distal half is imperfectly 

 divided by a groove into anterior or radial, and 

 posterior or ulnar portions. Its proximal end is 

 hollowed out to articulate with the lower end of 

 the humerus, and so form the elbow-joint, be- 

 hind which it projects backwards as the olecranon 

 process. 



c. The wrist consists of six small carpal bones arranged 



in two rows, proximal and distal, each row having 

 three bones. 



