SOURCE OF ENERGY 1 95 



steam engine, but it will be necessary to point out some 

 differences and show how the animal engine works. 



451. In the first place, the animal "fire box" is not the 

 stomach or lungs or any similar organ. The combustion takes 

 place in the cells and each individual cell of the animal body is 

 a unit so far as this process is concerned. Some cells require 

 more fuel than others in proportion as some are greater workers 

 than others. Consequently the muscle cells require much fuel. 

 But how does the food get to the muscle cells? That is another 

 question, and to make it clear we will return to the case of the 

 amceba. 



452. Digestion. When the amceba comes in contact with a 

 particle of food its protoplasm flows around the particle until 

 it is entirely enclosed and lies embedded in the protoplasm, 

 but with the particle there is also engulfed a droplet of water. 

 This is called a food vacuole. The water in which the amceba 

 lives is always slightly alkaline and the protoplasm of the amceba 

 is also alkaline, but if delicate test is made it is found that the 

 water or fluid of the food-vacuole becomes slightly acid and 

 soon changes may be observed in the food particle. If it 

 was a living object it soon dies; if it was a blue green 

 alga, the blue color is rapidly diffused into the surround- 

 ing medium. The vacuole becomes alkaline, and the food 

 substance becomes translucent as though it were being 

 dissolved, and gradually it disappears. Probably some por- 

 tions remain unchanged. The vacuole grows smaller until the 

 unchanged portions of the ingested object are closely surrounded 

 by protoplasm. Finally, what remains in the food vacuole is 

 ejected. While this has been going on, other food vacuoles 

 have been formed and the same series of phenomena take place 

 in each. All the while the animal is growing larger at the ex- 

 pense, evidently, of the substance of the food- vacuoles. It is 

 important to note that the food substance of the vacuoles 

 disappears and later reappears as protoplasm. Between these 



