32 AN INTRODUCTION TO ZOOLOGY 



inner ends of the coracoid and pre-coracoid are joined by a narrow 

 strip of cartilage, the epicoracoid, and the two epicoracoids meet in 

 the mid- ventral line. Passing forward from these, also in the middle 

 line, is a short bony rod, the omosternum, and in front of this is a 

 thin circular plate of cartilage, the episternum. Behind the 

 epicoracoids is a bony bar, the sternum or mesosternum, and posterior 

 to this again is a bilobed cartilaginous expansion, the Xiphisternum. 

 It has been pointed out previously that in addition to supporting 

 the fore-limbs and giving attachment to their muscles the ventral 

 part of the pectoral girdle forms a protection for the heart. Although 

 single in the adult, the various sternal elements arise from the paired 

 rudiments. It is not yet clear how far they correspond to the breast 

 bones in man, for they arise in connection with the coracoid bones, 

 whereas in man the sternum is developed in relation to the ventral 

 ends of the ribs. 



The bones of the hind-limb are very similar to those of the 

 fore-limb and homologous with them. The bone of the thigh is the 

 femur, a long slightly curved bone with a well-marked rounded head 

 and the distal end expanded laterally. The leg contains the tibio- 

 fibula, and the well-marked groove running along this indicates its 

 origin from two separate bones. In the tarsus are two rows each 

 of two bones. The proximal two are quite long and united together 

 at both ends. The pre-axial bone is termed the astragalus, and the 

 slightly larger post-axial one the calcaneum. The distal tarsalia 

 are quite small and easily overlooked. Unlike the hand, five com- 

 plete digits are present in the foot, and there is also a small additional 

 toe. Each digit has a well-developed metatarsal, and the number of 

 phalanges is two on the first and second toes, three on the third and 

 fifth, and four on the fourth. The extra toe, or calcar, consists of a 

 very short broad metatarsal on the pre-axial side of the hallux or 

 big toe, and sometimes also indications of a phalanx. Supporting 

 the calcar is a small bony nodule that may represent a displaced 

 tarsal. 



The hind-limb is suspended by the pelvic girdle In its primitive 

 condition this girdle, like the pectoral, forms an incomplete hoop 

 around the posterior part of the body. It has been modified in the 

 frog in accordance with the jumping habits of the animal, and the 

 ventral end has rotated through an angle of nearly 90 degrees, so 

 that instead of passing to the ventral side it is dorsal and almost in 

 a line with the backbone. The conspicuous hump on the back of the 

 frog marks the point of attachment of the pelvic girdle to the back- 

 bone. The girdle is similar in shape to the " wish-bone " or merry- 

 thought of a bird, and it exhibits on each side a deep cup-shaped 

 hollow, the acetabulum, into which the head of the humerus fits 



