12 LABORATORY STUDIES IN ZOOLOGY 



the arm bones. This socket or articuhiting excavation in the 

 girdle is known as the glenoid fossa. In the middle line of the ven- 

 tral portion the caudally projecting bony element is the sternum. 



The skeleton of the arm consists of the humerus in the upper 

 arm. In the forearm there are two bones side by side, the radius 

 on the thumb side and the uhia on the other side, as in man. 

 In the frog the radius and ulna are fused into one solid bone, the 

 radio-ulna. Distad of (beyond) the lower arm is the wrist or 

 carpus, containing six small bones known collectively as carpals, 

 arranged in two rows. The proximal row, situated at the end 

 of the radio-ulna, includes two elements; the distal row, three. 

 The sixth element is located on the inner side of the wrist. The 

 hand comprises five metacarpal bones, which support the palm 

 and at their ends articulate with the fingers. The first digit 

 (thumb) is absent. Its metacarpal is small. 



The humerus is a good example of a long bone serving as a 

 lever. The enlarged ends formed of cartilages are known as the 

 epiphyses, the shaft being the narrower portion between. The 

 proximal end of the humerus rests in the glenoid cavity, forming a 

 ball-and-socket joint, and is held in position by muscles and liga- 

 ments. The strong ridge on the proximal portion of the shaft is the 

 deltoid ridge and serves for the attachment of muscles. 



The distal end of the radio-ulna displays very obvious traces of 

 its double origin. The proximal end is hollowed out for articula- 

 tion with the humerus, and the ulnar part is produced proximally 

 to form a projection, the olecranan, which fits over the rounded 

 end of the humerus at the elbow. 



The pelvic girdle is shaped like a V with the open part directed 

 cephalad. On each side the arm of the V is a distinct bone, the 

 ilium. The ilium of each side articulates with the transverse 

 process of the ninth vertebra. The ilia fuse with one another 

 at the caudal end where there is formed a circular platelike por- 

 tion excavated at its center. The cephalic half of this plate is 

 formed by the fused ilia. The caudal margin is formed by the 

 ischia (singular ischium) one on each side, which have joined at 

 the middle line. The ventral margin of the plate is occupied 

 by the puhes (singular, pubis) which like the ischia have joined 

 at the middle line to form a triangular mass of cartilage. In old 

 frogs this element becomes calcified. The ilium, ischium, and 

 pubis all take part in the formation of the acetabulum, the excava- 

 tion in which the leg skeleton articulates with the girdle. 



