XII. IO 



PELVIC GIRDLE 



313 



the femur articulates. The girdle thus provides a plate to which the 

 muscles that brace the limb can be attached in such a way as to 

 balance the body on the leg. 



In urodeles the pelvic, like the pectoral, girdle becomes reduced and 

 mainly cartilaginous. The pelvic girdle of anurans is highly specialized 

 and unlike that of any other vertebrate. The ilia are very long and 

 directed forward to articulate with the transverse processes of the single 

 pair of sacral vertebrae. The base of the ilium is expanded to make the 



ac. Cryptobranchus 



is. it- % 



Eryops 



Necturus p. 



Iguana 



Fig. 184. Pelvic girdles of lower tetrapods. Regions mainly cartilaginous are stippled. 

 ac. acetabulum; /'/. ilium; is. ischium; of. obturator foramen; p. pubis. (After Evans.) 



dorsal portion of a disk, of which the pubis is the anterior, the ischium 

 the posterior part, with the acetabulum at the centre. The girdle is 

 thus developed into a long lever for transferring force from the limb 

 to the vertebral column during jumping. 



Considerable movement is possible at the ilio-sacral joints, at least 

 in Salientia (Whiting, 1961). In Rana the ilia may rotate through an 

 angle of over 90 on the sacral ribs in the vertical plane. This move- 

 ment is used during a strong leap. In Discoglossus the sacrum can be 

 turned laterally on the pelvis through 20°. The movement is used both 

 in turning to take food and in locomotion. In Xenopus the sacrum 

 can slide backwards and forwards on the pelvis, producing a con- 

 siderable shortening and lengthening of the whole animal. This 

 movement is probably used in driving into the mud. 



10. The limbs of Amphibia 



The pattern of bones and muscles in fore and hind limbs of tetrapods 

 is surprisingly constant in spite of the various uses to which the limbs 



