130 



COMPARATIVE ANATOMY 



7 



well as the entire pectoral arch and sternum, are extraordinarily 

 developed. In the Ratit^, 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. 



Claws were present on the terminal phalanges 

 of all three digits in Archaeopteryx. 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 elements, 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 

 radiale (including, no longer possesses any distinct tarsal ele- 

 according to Em- men ts, though, as in Chelonians and Lizards, 



ery, a carpal of the xl ~ i , i 



prepollex); u, ul- tne * 00 "k really moves by an intertarsal articu- 



nare; C, centrale ; lation. Of the original five metatarsals, the 



l to 5, the five car- fifth soon disappears, while the second, third, 

 palia, as yet unossi- 1^-11 -1-^1 



fied, of which l and an d fourth become united with one another 



2, as well as 3, 4, and with the distal element of the tarsus 

 and 5 have become to f orm a single bone, the tarsometatarsus 



fused together ; t, /-p.- m -, -f o -r>\ mi n 



pisiform T / to V, (Figs. Ill, 112 B). The first metatarsal 



the metacarpals. remains to a greater or less extent inde- 

 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.) 



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 



''" 



Alligator lucius. 

 (From above.) 



R, radius ; U, ulna ; r, 



