482 



COMPAEATIVE ANATOMY. 



first and fhird one only {pi^")'> *^e first and second fingers rarely 

 have an extra phalanx. 



FuRBRiNGER, M., Die Znoclien und Muskeln der Extremitaten bei den schlan. 

 genartigen Sauriern. Leipzig, 1870. 



§365. 



The greater variation in adaptive relations to various conditions 

 is implied by the greater variations in the structure of the skeleton of 

 the Mammalian forelimb. Its elements somewhat resemble the 

 lower condition, such as is seen in the Chelonii, with regard to _ the 

 number of the carpal bones. Although the manus is often modified 

 by the atrophy of certain fingers, the extremity, even in the lower 

 divisions of the Mammalia, has very various uses. Owing to the 

 greater power of movement possessed by the two bones of the fore- 

 arm, and the connection between one of them (the radius) and the 

 manus, the anterior extremity loses its lower function of an organ 

 of support, and is converted into a prehensile organ. This phseno- 

 menon is seen in the Didelphia, as well as in the Monodelphia ; it is 

 most complete in the Primates. The carpus has the three primitive 

 pieces of the proximal row. A centrale, also, is not unfrequently 

 present (Rodentia, Insectivora, Lemurs, Orang, and, for a short 

 time, Man). The distal row of the carpus have the two ulnar bones 

 fused into an uncinate (cf. Fig. 268, I 11). The pisiform is n, 

 special bone, which is attached to the ulnar edge of the carpus ; it 

 is very large in many forms. It is also found in 

 the Eeptilia, and may be shown to be the soli- 

 tary remnant of a numerous series, which was 

 possessed by the Eualiosaurii. 



The modifications derived from this series of 

 forms are very closely correlated with the function 

 of these parts. When the arm is used as an organ 

 of flight (Chiroptera), we find that its different 

 portions are considerably elongated; and so, 

 again, they are shortened, and various parts be- 

 come very large in those numerous cases in 

 which the arm acquires a special function, as in 

 digging and so on ; the Monotremata, many 

 Edentata, Talpa, etc., are examples of this. In- 

 stead of this great increase in size, which is seen 

 in various parts of the skeleton of the arm, there 

 may be atrophy, as is the case with the fore-limb 

 of the Cetacea. It forms a paddle, the separate 

 parts of which have but little power of move- 

 ment, and the various bones of these parts may 

 lose all their articulations, and become united into an unjointed 

 fin-like mass (Fig. 267). 



In another series, several of the fingers are atrophied, and the 



Fig. 267. Anterior 

 extremity of a young 

 Dolphin, s Scapula. 

 h Humerus, r Ea- 

 dius. 11 Ulna, c Car- 

 pus. m Metacarpus. 

 p Phalanges. 



