THE ENDOSKELETON 711 



We have already seen that in the frog the radius and ulna as well 

 as tibia and fibula are fused together, while the tarsals are considerably 

 elongated. Such fusion is not uncommon in many of the animals. The 

 extent of fusion varies, however, considerably. In the reptile limb there 

 is an intratarsal joint, so that the motion of the foot upon the leg lies 

 between the two rows of tarsal bones, instead of between the tarsals and 

 the bones of the shank. This is quite similar to the condition in birds. 



Although limbs are lacking in the snakes and in some of the lizards, 

 there is nevertheless a considerable increase in the number of phalanges 

 in those reptiles where limbs do occur, while the more proximal bones 

 shorten. In some of the ichthyosaurs there may be as many as a hun- 

 dred phalanges in a single digit. 



The skeleton of pterydactyls shows the fifth digit greatly developed, 

 which forms a definite support for the wings, while the other digits re- 

 main more or less normal. In birds the wings are considerably modified 

 (Fig. 420), although the structure is practically normal, up to the region 

 of the carpus, the carpal bones being greatly reduced by fusion, while 

 the metacarpa's and digits, no matter what their modification, are only 

 three in number. 



Embryological studies of the chick show us that, although the first 

 digit begins to develop, it is entirely lost, and the fifth metacarpal, which 

 is present in the embryo, fuses early with the fourth, so that the digital 

 formula is II, III, IV. Added to this, there is an extensive fusion of 

 the bones of the tarsus and foot ; the ankle joint is intratarsal, the basal 

 row of tarsal bones fuses with the tibia, while the fibula is considerably 

 reduced to form the tibio-tarsus. The tarsales unite in the same way 

 with the fused metatarsals to form the tarso-metatarsus. 



There are hardly ever more than four toes, but the number of 

 phalanges increases from two in digit II, to five in digit V. Ostriches 

 only have two toes, and many other birds three. In the mammals, 

 especially in the higher forms, there is considerable motion of both hand 

 and foot, rotation in the hand being especially noticeable by the motion 

 of the lower end of the radius around the u}na. In the whales, the basal 

 part of the forelimb is greatly shortened, while there is considerable 

 multiplication of the phalanges. The hind limb is entirely lacking in 

 some whales, while in others, there are two vestigeal bones supposed to 

 be the femur and tibia, imbedded in the muscles of the trunk. 



A supra- or entepicondylar foramen frequently perforates the inner 

 lower end of the mammalian humerus, while in many forms the ulna is 

 fused with the radius in varying degrees. However, the ulna, whether 

 fused or not, always has on its proximal end a strong olecranon process 

 which extends beyond the elbow joint for the attachment of the exten- 

 sor muscles of the forearm. 



The earliest prominences for the attachments of the muscles on the 

 femur are known as trochanters. They vary from one to three. The 



