j^ THE FROG. PHYLUM CHORDATA 



there are in connection with it no unpaired structures Hke the 

 stern inn. The greater part of each half consists of a long slender 

 bone, the hip-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 



Pb 



Fig. 273. — Side view of frog's pelvis. — From Thomson, after Ecker. 

 //., Ilium ; Is., ischium ; Pb., pubis ; Ac, acetabulum. 



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. Ventrally, between the ilium and the ischium, lies a 

 triangular piece of calcified cartilage, the pubic 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. 



LIMBS 



The upper arm contains a single bone, the humerus. This 

 consists 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 

 irregular in shape and serves for the articulation of the forearm 

 bone or radio-ulna, which bears a groove showing its origin from 

 two bones, an inner radius and an outer ulna. The upper end 

 of the radio-ulna is hollowed to receive the humerus at the elbow- 

 ioint, behind which it projects as the elbow-bone or olecranon 

 • "cess. The wrist consists of six small carpal bones arranged 

 ^ rows across the limb. Those of the first row are named 



