24 MANUAL OF PHYSIOLOGY. 



performed by the agency of the living cell-elements, in which we 

 can recognize independent manifestations of life, such as the re- 

 spouse to stimuli, motion, nutrition, growth, etc. The living ac- 

 tivity of organisms requires for its perfect development certain 

 external conditions, namely, a certain degree of warmth and 

 moisture. Without heat and moisture the chemical interchanges 

 just mentioned cannot go on, and the organism is either destroyed 

 or remains in a state of inactivity. 



The nutrition of the animal body which is accomplished by 

 means of the processes of assimilation already mentioned enables 

 it to grow, and, up to a certain point, increase in size, and further 

 to undergo many changes in form and texture. There is, how- 

 ever, a limit to this assimilative power: nutrition loses in activity, 

 growth gradually stops, and after a time decay appears and is fol- 

 lowed by death. 



Thus organisms exist only for a limited period of time, during 

 which their size, form, and functional activity are constantly 

 undergoing some alteration dependent on the incessant changes 

 in their molecular construction. 



This cycle of changes through which organisms pass we speak 

 of as their lifetime. During this lifetime, at the period when 

 their functional activity is at its height, they possess the remark- 

 able faculty of producing individuals like themselves. 



This is accomplished by setting apart a cell which, under favor- 

 able circumstances, assumes special powers of growth, increases in 

 size by the rapid formation of new cells, and develops into an 

 independent living unit. In time it arrives at maturity, and be- 

 comes like its parent, and then passes through the same cycle 

 by its power of assimilation it grows to maturity, reproduces its 

 like, decays, and dies. 



