48 THE FROG CHAP. 



Connected with the median ventral portion of the shoulder- 

 girdle is the sternum, or breast-bone, which consists of two 

 separate parts, one extending forwards, the other backwards, 

 in the middle line, and each formed of a flattened bony rod 

 (Ep, Sf], tipped with a flat plate of cartilage. 



The anterior bony rod with its terminal cartilage is called 

 the omosternum (Ep) ; the posterior bony rod is the stet num (St ), the 

 bilobed cartilage at its end the xiphisternum (Kn ). The cartilages 

 uniting the inner or ventral ends of the clavicles and coracoids are dis- 

 tinguished as the epicoracoids ( Co'). 



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 procoracoid, 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 a humerus which has not been 

 allowed to dry you will see that the shaft (Fig. 13, sti) 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 

 extremity. 



