130 



COMPARATIVE ANATOMY 



well as the entire pectoral arch and sternum, are extraordinarily 

 developed. In the Ratitae, however, the wing has undergone 

 regressive changes in connection with the habits of these Birds. 



Of the six or seven carpals which may be present in the embryo, 

 the three distal become fused with the corresponding metacarpals, 

 thus forming a carpometacarpus (Figs. Ill, 112 A), and in the 

 adult only the two proximal remain separate as a radiale and an 

 ulnare. The three metacarpals themselves 

 become united together proximally, and the 

 second and third distally : they only bear a 

 very limited number of phalanges at their 

 free ends. 



Claivs were present on the terminal phalanges 

 of all three digits in Archseopteryx. In certain 

 recent Birds the first digit bears a claw, and more 

 rarely the second and even the third also. ■ 



The tarsus is still more reduced in Birds 

 than in Reptiles, and consists in the embryo 

 of three elernents, two small proximal and a 

 broader distal. The former (tibiale and 

 fibulare) unite later with the distal end of 

 the tibia, thus forming a tibiotarsus, while 

 the latter, which corresponds to tarsalia / to 

 V, becomes included in the base of the 

 metatarsus. Thus the foot of adult Birds 

 no longer possesses any distinct tarsal ele- 

 ments, though, as in Chelonians and Lizards, 

 the foot really moves by an intertarsal articu- 

 lation. Of the original five metatarsals, the 

 fifth soon disappears, while the second, third, 

 and fourth become united with one another 

 and with the distal element of the tarsus 

 to form a single bone, the tarsometatarsus 

 (Figs. Ill, 112 B). The first metatarsal 

 remains 

 pendent, 



The number of toes varies between two (Struthio) and four; 

 that of the phalanges is normally 2, 3, 4, 5, reckoning from the 

 first to the fourth digit. The tibia, even from the first, greatly 

 exceeds the fibula in size, and the two bones become fused to- 

 gether distally. 



In both Jimbs the bones are usually pneumatic. (See under 

 Air-sacs.) 



Fig. 110.— Right Car- 

 pus OF A Young 

 Alligator Indus. 

 (From above.) 



i?, radius ; IT, ulna ; r, 

 radiale (including, 

 according to Em- 

 er\', a carpal of the 

 prepoUex) ; u, ul- 

 nare ; C, centrale ; 

 1 to 5, the five car- 

 palia, as yet unossi- 

 fied, of which 1 and 

 2, as well as 3, 4, 

 and 5, have become 

 fused together ; f, 

 pisiform ; I to V, 

 the metacarpals. 



to a greater or less extent inde- 



Mammals. — In Mammals the anterior extremity either re- 

 mains in the condition of a simple organ of locomotion, serving for 

 progression on land ; or it may become modified in adaption to an 



