10 INTRODUCTION TO GENERAL PHYSIOLOGY 



The difficulty will probably occur to the reader that solid 

 particles, such as the bacteria and algre used for food, enter an 

 amoeba, although solutions of dyes do not. The difference is due 

 to this very fact of the latter being solutions. They can do no 

 mechanical violence to the membrane, whereas a solid particle 

 breaks through. The hole in the membrane, however, is mended 

 as soon as the particle has passed. What happens is like the 

 dropping of a needle through a soap film. When the point touches 

 the film, it becomes covered with a continuous film, which prevents 

 an actual break. As the needle passes through, before the eye-end 

 has left the film, it also has a film over it, which is left behind as 

 the needle drops through. 



It may be pointed out here that the substances resulting from 

 the digestion of the food of the amceba are freely dissolved by 

 water. They would quickly be washed out if they could pass 

 through the membrane, and so be lost to the organism. 



In order to understand how such a membrane could be formed, 

 we must direct our attention to the doctrine of energy, especially 

 in certain aspects. This is, in any case, a necessary preliminary to 

 further study. 



Energy and its Laws 



Living beings are always doing something, making changes 

 in their surroundings. This means work. When we have done 

 work, we feel that we have lost something that has enabled us to 

 do the work. Now, this is called "energy," and is actually 

 defined as the capacity of doing work. Moreover, it can be 

 accurately measured and shown to be exactly equal to the work 

 done. Since energy is not a thing to be seen, apart from the 

 material bodies possessing it, we are rather apt to overlook its 

 importance in ordinary life. The chief use of the food we take is 

 to supply us with energy. If it were merely to make body- 

 substance, flesh or bones, we should need very little indeed. A 

 certain quantity of any particular food-stuff contains a definite 

 amount of energy, no more and no less, and will enable a certain 

 amount of work to be done, no more and no less. 



The reader is familiar, no doubt, with the two great laws at the 

 foundation of the doctrine of energy. They are usually known as 

 the First and Second Laws of Thermodynamics ; but it would be 

 better, especially from our physiological point of view, to speak of 

 them as laws of " Energetics," since they apply to all forms of 

 energy. They were first established by investigations of that form 

 of energy known as heat, hence their usual designation. 



The words, " forms of energy," just used, imply that energy may 



