THE HUMAN ORGANISM 



273 



ourselves well and fit and (2) to enable us to cooperate with 

 others in keeping the community well and fit. For many cen- 

 turies even physicians were 

 unable to become thor- 

 oughly acquainted with the 

 structure and workings of 

 the human body, because 

 (i) popular superstition 

 stood in the way of dissect- 

 ing, or cutting apart, bodies 

 for the purpose of training 

 physicians and surgeons ; 

 and (2) almost complete 

 reliance upon authority in 

 all studies left students 

 satisfied to learn what the 

 masters handed down from 

 generation to generation, 

 instead of trying to learn 

 facts at first hand. Today 

 we are able to get more reliable information from books, charts, 

 manikins, and models, and more direct knowledge from a 



Fig. 121. 



The leopard frog [Rana 

 virescens) 



This common batrachian shows the general 

 external plan of structure among vertebrates: 

 a head with a mouth and most of the sense 

 organs, and a trunk with two pairs of limbs. 

 (Courtesy of .American Museum of Natural 

 History) 



Fig. 12 2. Homology in hind legs of vertebrates 



The hind legs of all four-footed animals and the legs of birds correspond to the paired 



hind fins of fishes. In spite of the many differences between them we can easily find 



the resemblances in the legs and feet of the frog, a; the lizard, b; the bird, c; the 



horse, d; the rhinoceros, c: and the elephant, /. H is the heel in each case 



study of prepared material and from dissections of other 

 animals. It is remarkable how much we can learn even from 



