416 



COMPAEATIVE AN-ATOjMY. 



division undergoes differentiation ; a number, generally not more 

 than five, terminal joints can be distinguisbed as fingers and toes. 

 As a part of tbe body wbicb generally projects towards the exterior 

 is more under modifying influences than any other part, we find a 

 large number of adaptations in them ; few parts of the body present 

 so many metamorphoses as these terminal parts of the appendages — 

 the hand and foot. 



The primitive union of the fingers or of the toes into a swim- 

 ming-plate, represented by the hand and foot, is retained in the 

 natatory membrane of many Reptiles, in the hind-limbs of many 

 Birds, and also in a number of Mammals, where it is always con- 

 nected wath the adaptation of these appendages to the function of a 



swimming organ. 



The angulation of the limb at- 



tainedto,in connection with terres- 



A 



^ 



i^x: 



B 



^ 



Fig. 215. Diagram to show the differen- 

 tiation and alteration in the direction of 

 the axes of the limbs in the Vertebrata. 

 .4 Fish. B Amphibian (the side-view, 

 which one has been obliged to give so as 

 to compare it with the rest, gives to this 

 and to the next figure the appearance of 

 tlse body being raised up). C Reptile. 

 D Mammal, a Shoulder girdle. ^Pelvic 

 girdle. 



trial locomotion, and which is also 

 advantageous in aquatic locomo- 

 tion, becomes gradually different 

 in the case of the two extremities; 

 the difference corresponds to the 

 functions performed by the an- 

 terior and posterior extremities 

 when moving about on land. 



These relations are distinctly 

 seen even in the Amphibia (B) ; 

 but the difference in position 

 between the upper and forearm, 

 and thigh and leg, is not well 

 marked. The upper arm and 

 thigh are turned outwards to 

 almost the same extent. There 

 is a greater difference between 

 them in the Reptilia ((7), and this 

 is still more marked in the Mam- 

 malia, where the planes in which 

 the angles of the limbs of either 

 side are set are parallel to the 

 vertical median plane of the 

 body. This gives greater inde- 

 pendence to the limbs, which 

 have now become supports for 

 the body, as they raise it up 

 from the ground. Together with 



this change in the aspect of the 

 planes, in which the angle formed by the extremity lies, the angles 

 between the equivalent portions in each limb in the Mammalia 

 cease altogether to agree with one another (D) ; in fact they point 

 in an opposite direction in the case of the fore and hind limbs 

 respectively. The angle between the upper and forearm is open 



