14 NUTRITIONAL PHYSIOLOGY 



was originally foreign in its nature. We use the expression 

 in much the same sense when we speak of the assimilation 

 of immigrant peoples. The word nutrition is used in 

 about the same way as assimilation, the only distinction 

 being that we speak of the nutrition of cells (or cell- 

 aggregates), while we speak of the assimilation of food, 

 the former term referring to the structure nourished and 

 the latter to the supplies worked over for the purpose. 

 The word digestion is best restricted to the preliminary 

 stages of the assimilation process; its application will be 

 defined later. 



Respiration has been said to be a process in course of 

 which compounds are decomposed that their potential 

 energy may be made available. The greater part of the 

 released energy appears as heat. A smaller part mani- 

 fests itself in movements through which resistances are 

 overcome. The facts in regard to the production of 

 energy are naturally better known for the larger animals 

 than for the free-living cells, but " the whole is equal to 

 the sum of its parts." The energy from respiration may 

 in exceptional cases become kinetic, in the form of light 

 or electric discharge (firefly, electric eel). The energy 

 which shows itself in movement is of particular interest 

 to us. Motion, when exhibited by animal cells, is almost 

 always the expression of contraction, the word being used 

 in a physiologic sense. So used, contraction does not mean 

 diminution of volume, but does mean diminution of sur- 

 face and active shortening in one or more dimensions. 

 Although an increase in other dimensions attends such 

 changes of form, we do not talk of the " expansion " of 

 cells. It is the contraction which is the positive and 

 forcible element in the movement. When this is said we 

 intentionally leave out of account some types of movement 

 occurring commonly in the plant world, in course of which 

 the cells actually change their volume through gain or loss 

 of water. Among free-living cells the type of move- 

 ment may be " ameboid," that is, a flowing of the cell 

 contents to conform to an ever-changing outline. Con- 



