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, St), tipped with a flat plate of cartilage. 



The anterior bony rod is called the episternmii (Ep), its terminal 

 cartilage the oinosternuni ; the posterior bony rod is the sternum (SI. ), 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 cartilage-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. ii); 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, 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 hone-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. 



