1 8 INTE OD UCTION. 



and the clearer will grow his insight into the causes and tendencies 

 of the processes of which they are an expression. 



In all his studies he must seek not merely to train his powers 

 of observation ; he must endeavor to cultivate his ability to inter- 

 pret what he sees ; to deduce the processes and causes that have 

 wrought the results he perceives, and to compare those deductions 

 with the conceptions of living things he has already formed, so that 

 his ideas may remain in perfect accord with one another as his grasp 

 of the subject enlarges. By so doing he may hope to create a life- 

 like mental picture of the body both in health and during disease. 



The activities of the body involve changes in the substances of 

 which it is composed. Some of these changes are always destruc- 

 tive in character that is, they result in chemical rearrangements 

 which convert more complex combinations of less stable nature into 

 simpler combinations of greater stability. Such chemical changes, 

 whether they take place within the body or in external nature, 

 among organic or inorganic substances, are always accompanied by 

 a liberation of energy hitherto locked up or stored in latent or 

 potential form in the compounds of higher complexity. It is this 

 liberated or kinetic energy which is utilized by the bodily mechan- 

 ism for the performance of internal or external work. When 

 directed in various ways and operating through different structures, 

 this energy occasions visible movement, appears as heat, etc., or 

 passes again into the latent form in the elaboration of more com- 

 plex chemical substances from those of simpler constitution. 



These associated transformations of matter and energy involve a 

 continual loss to the bodily economy. The stock of energy is dim- 

 inished during the execution of external work and by the dissipa- 

 tion of heat. The store of useful chemical substances is reduced 

 by their progressive conversion into compounds that are insuscep- 

 tible of further utilization, and which, in many cases, may act injuri- 

 ously upon the structures of the body. Under normal conditions 

 such substances are eliminated from the body. 



It is evident, then, that the body is constantly suffering a loss of 

 both energy and matter. This loss must be made good if the 

 activities of the body are to be maintained, and this is accomplished, 

 during health, through the absorption of fresh material, containing 

 latent energy, from the food taken into the body. 



The activities of the body are not the same in all its parts. They 

 are all alike in one particular namely, that each part must main- 



