48 THE FROG CHAP. 



consists of two separate parts, one extending forwards, 

 the other backwards, in the middle line, and each 

 formed of a flattened bony rod (st, f st"), tipped with a 

 flat plate of cartilage. 



The anterior bony rod with its terminal cartilage is called 

 the omosternum (st'), the posterior bony rod the sternum (st), 

 and the bilobed cartilage at its end the xiphisternum (st"). 

 The cartilages uniting the inner or ventral ends of the 

 clavicles and coracoids are distinguished as the epicoracoids 

 (Fig. ii, ep. cor.). 



All the bones of the shoulder-girdle and sternum are 

 replacing bones except the clavicle. This can be removed, 

 and is seen partly to surround a bar of cartilage, the pro- 

 coracoid, which stretches between the scapula and the 

 epicoracoid and is ordinarily completely concealed by the 

 clavicle. 



The Fore-limb. The upper arm is supported by a 

 single bone, the humerus (Fig. 8, HU), the first example 

 we have had of what is conveniently called a long 

 bone. It consists of a roughly cylindrical shaft, formed 

 of dense bone, and of two extremities the proximal of 

 partially calcified cartilage, the distal of spongy or 

 cancellated bone. The proximal extremity or head is 

 convex, and fits into the glenoid cavity of the shoulder 

 girdle (Fig. n) ; the distal extremity or condyle is almost 

 globular, and is articulated with the bone of the fore- 

 arm. 



In a longitudinal section of the humerus which has 

 not been allowed to dry you will see that the shaft 

 (Fig. 13, A, sh) is not a solid rod, but a tube, containing a 

 cavity, the marrow-cavity. In this way the weight of the 

 bone is diminished without its strength being impaired. 

 The marrow-cavity contains a substance called bone- 

 marrow, composed chiefly of connective-tissue and fat, 

 with blood-vessels. The proximal end of the hollow 

 shaft is, as it were, plugged by the cartilaginous ex- 

 tremity. 



