OUK BODILY FBAME. 29 



with the liver in digestion ; so are, likewise, the rami- 

 fication of the muscular system, the peculiar structure 

 of the urinary and sexual organs, and so forth. In 

 all these anatomical relations man is a true vertebrate. 



Aristotle gave the name of four-footed, or tetrapoda, 

 to all the higher warm-blooded animals which are 

 distinguished by the possession of two pairs of legs. 

 The category was enlarged subsequently, and its title 

 changed into the Latin " quadrupeda," when Cuvier 

 proved that even " two-legged " birds and men are 

 really " four-footed " : he showed that the internal 

 skeleton of the four legs in all the higher land- 

 vertebrates, from the amphibia up to man, was 

 originally constructed after the same pattern out of a 

 definite number of members. The " arm " of man and 

 the " wing " of bats and birds have the same typical 

 skeleton as the foreleg of the animals which are con- 

 spicuously "four-footed." 



The anatomical unity of the fully-developed skeleton 

 in the four limbs of all tetrapods is very important. 

 In order to appreciate it fully one has only to compare 

 carefully the skeleton of a salamander or a frog with 

 that of a monkey or a man. One perceives at once 

 that the humeral zone in front and the pelvic zone 

 behind are made up of the same principal parts as in 

 the rest of the quadrupeds. We find in all cases that 

 the first section of the leg proper consists of one strong 

 marrow-bone (the humerus, in the f orelimb ; the femur, 

 behind) ; the second part, on the contrary, originally 

 always consists of two bones (the ulna and radius, in 

 front ; the fibula and tibia, behind). When we further 

 compare the developed structure of the foot proper we 

 are surprised to find that the small bones of which it 

 is made up are also similarly arranged and distributed 



