PAIRED LIMBS 559 



as all but the smallest birds use their legs alternately, the only 

 bipedal mammal which does so is man. In walking his hind-limbs 

 are plantigrade, but his peculiar foot allows digitigrade running 

 (Fig. 435). His fore-limbs (Fig. 349) are remarkable for opposa- 

 bility of the thumb and for the degree of twisting of which the 

 forearm is capable. These features, perhaps originally useful 

 for climbing trees, are generally considered to have assisted in 

 the development of man's brain by allowing him to grasp and 

 inspect solid objects, and so to have more to think about. The 

 only vertebrates except the primates which can handle things in a 

 comparable way are some rodents, notably the squirrel, and a few 

 birds, especially the parrots. The last have much larger cerebral 

 hemispheres in proportion to their size than any other birds. 



Extreme modification of limb structure is found, as would be 

 expected, in those mammals which have ceased to walk on the 

 surface of the land. In the toothed whales there is no trace of 

 any skeleton of the hind-limb, and in the whalebone whales there 

 are only small internal bones representing at most femur and 

 tibia. The fore-limb in both groups has become a flattened paddle, 

 with no external trace of digits. The bones are reduced in length, 

 and packed tightly together, so that there is some resemblance 

 to the condition in the dogfish (Fig. 376). There is much irregu- 

 larity in the arrangement of the carpals, and their homology is 

 difficult. Hyperphalangy— an increase in the number of phalanges 

 above the normal — is usual. In the bats the whole fore-limb is 

 lengthened, and the wing is supported chiefly by the middle 

 phalanges of digits I, III, IV, and V in the fruit-eating bats, 

 or the metacarpals of II to V in the insect-eating forms (Fig. 552). 

 The ulna is almost lost. The hind-limbs are short, and only the 

 digits are free. 



LIMB GIRDLES 



The proximal bone of each hmb articulates with a special 

 inner part of the skeleton, called a girdle, pectoral for the fore- 

 limb and pelvic for the hind. These girdles are generally considered 

 to be derived from the anterior basals of the protovertebrate 

 fins, which have sunk inwards and become enlarged. In addition, 

 in the pectoral girdle only there may be a superficial series of 

 dermal bones, which are probably only secondarily associated 

 with the hmbs. In fishes, the endoskeletal girdle is ususally 

 relatively simple ; there is a single cartilage or bone, or sometimes 



