12 INTRODUCTION 



in material from outside and building it into the bodily 

 structure, that is, the making of complex chemical com- 

 pounds out of simpler ones ; and the breaking down, or 

 reducing to simpler forms, of compounds already formed. 

 The former results in growth or repair of cell substance, 

 and is the storing up of energy ; the latter is the setting 

 free of energy, and the production of waste material to be 

 removed as no longer valuable. Nutrition includes all 

 the chemical changes which take place in living matter. 



The other essential property of living bodies is the 

 power of reproduction, or of giving rise in some way to 

 living beings like themselves. 



The first of these properties, nutrition, belongs to every 

 individual cell, to every plant, and every animal, as neces- 

 sary to its own continued existence. The second, repro- 

 duction, is needful only for the continued existence of the 

 race, and is in some cases possessed only by certain indi- 

 viduals of the race. Single cells are, however, capable of 

 giving rise by self-division to other cells like themselves. 



Life is sustained by the ceaseless exercise of the two 

 powers of nutrition and reproduction. 



7. Other Properties of Living Cells. Certain other 

 properties are found to exist in most cells in the body, 

 for example, in the white corpuscles of human blood, 

 which are clearly defined nucleated cells. 



These are contractility, or instability, that is, the power 

 of changing form without the application of pressure ; 

 irritability, or the power of vigorous action under stimulus, 

 as, for instance, when the blood cells contract under the 

 influence of electricity ; conductivity, or the power of pass- 

 ing on to distant parts of the cell the influence exerted 

 by a stimulus upon a single point; and coordination, or 

 the capacity in all the parts to work together in definite 



