THE HUMAN ORGANISM 



275 



Oall bladder 

 Liver ^' 



Pancreas 



Jnlestine 



Kidney 



carving poultry or from studying the joints of meat and other 

 m.aterials that come from the butcher shop. 



224. General plan of vertebrates. On page 16 we started to 

 make a comparison between the human body, representing a 

 vertebrate, and an insect body. 

 The frog is a convenient ani- 

 mal for showing the general 

 plan of vertebrates. We note 

 the main body, or trunk, with 

 the head at one end and with 

 two pairs of appendages. In 

 the more familiar groups of 

 vertebrates (mammals, birds, 

 amphibians, and certain rep- 

 tiles such as lizards and tur- 

 tles) the posterior (hind) 

 appendages are so much alike 

 that we generally call them by 

 the same name — legs. To be 

 sure, the flippers of the seal or 

 the whale resemble the fins of 

 fishes more than they do the 

 limbs of land animals, and the 

 fins of fishes do not resemble 

 our legs or arms at all, except 

 perhaps in their positions on 

 the body; yet they are truly 

 homologous organs (see Figs. 

 122 and 123). The head is 

 easily recognized in all classes, 

 although it is not always on a 

 distinct neck. 



The trunk of the frog consists of a body wall inclosing a 

 large cavity. In the mammals, including man, the body cavity 

 is divided into two chambers by a muscular partition called the 

 diaphragm (see Figs. 84 and 124). Inside the cavity of the 



Fig. 125. The viscera of the frog 



In the body cavity of the frog are lo- 

 cated the principal breathing organs (the 

 lungs) ; the principal digestive organs 

 (stomach and intestines, liver and gall 

 bladder, and pancreas) ; the principal 

 excreting organs (the kidneys) ; the blood- 

 pumping organ (the heart); and the re- 

 productive organs (ovaries or testes). The 

 largest blood vessels, air tubes, and con- 

 necting ducts are also in this cavity 



