108 



COMPARATIVE ANATOMY. 



The number of toes varies between two 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. 



Palaeontological discoveries prove clearly that the form of the Bird's tarsus 

 has been gradually evolved from that of Dinosaurian-like forms. 



The foot of Archseopteryx was very similar to that of existing 

 Birds, though the primitive separation of the tarsal elements is 



FIG. 94. POSTERIOR EXTREMITY or BLACKBIRD (Tur dun mcrula}. 



Fe, femur ; T, tibia, united with the fibula (F) ; t,*, apophyses of the tibia and meta- 

 tarsus, corresponding respectively with the proximal and distal tarsal-plate : the 

 original division of the metatarsus (Mt) into separate bones is indicated at its 

 distal end at t ; / to 17, first to fourth digits. 



much clearer, and marked by deeper furrows. The first metatarsal 

 was turned slightly outwards, and the toe itself backwards, like 

 that of most existing Birds. 



Mammals. In Mammals the anterior extremity either 

 remains in the condition of a simple organ of locomotion, or it 

 gives rise to a prehensile organ. 1 In the latter case the radius and 

 ulna, instead of being firmly connected together, remain separate, 

 and articulate with one another. The movements of rotation 

 which are thus rendered possible are spoken of as pronation 

 and supination. 



1 In Bats, by the elongation of the fingers, between which a wing-membrane is 

 stretched, it serves for flight. 



