38 



MANUAL OF ELEMENTARY ZOOLOGY 



bone or ilium, corresponding in position to the scapular part 

 of the shoulder girdle, which runs downwards and backwards 

 from the sacral vertebra, curving inwards on the under side 

 of the body to join its fellow. The junction is enlarged' 

 into a flattened mass by the addition of several elements 

 which are more distinct while they are being formed in 

 development than they are in the adult. Behind lies a 

 ridge of bone known as the ischium, which consists at first 



of two parts, one belonging to 

 each half of the girdle. A slight 

 groove marks the limits of this 

 bone. In front a similar ridge, 

 not marked off from the ilium, 

 is known as the pubis, and re- 

 presents a pair of pubic bones 

 found in certain other animals, 

 though its relation to the bones 

 so called in man is uncertain. 

 Ventrally, between the pubis and 

 the ischium, lies a triangular 

 piece of calcified cartilage, the 

 postpubic cartilage. In each of 

 the flat sides of the mass formed 

 by the union of these structures 

 is a round hollow, the leg-socket 

 or acetabulum, into which fits 

 the head of the thigh-bone. 



The upper arm contains a 

 single bone, the upper arm 

 bone or humerus. This con- 

 sists of a stout shaft, swollen at each end, and bearing on 

 its inner side a ridge known as the deltoid 

 ridge. The swelling at the upper end is the 

 head, and fits into the glenoid cavity of the shoulder 

 girdle. That at the lower end, the trochlea, is more ir- 

 regular in shape and serves for the articulation of the 

 forearm bone or radio-ulna. In man, and in most animals 

 whose limbs are built upon the same plan as those of 

 the frog, the forearm contains two bones, the radius and 

 the ulna, and traces of the fusion of these can clearly 

 be seen in the frog. The radius is the inner of the 



Fig. 18. — The hip girdle of a 

 frog, seen from the left side. 



ac. t Acetabulum; z7., # ilium ; is., 

 ischium ; pb. , pubic region of 

 ilium ; pb 1 . , post-pubic cartilage. 



