32 MANUAL OF PHYSIOLOGY. 



tions, and the results., they give rise to in the various tissues, 

 practically make up the other part of physiology. 



3. Vital Phenomena. The so-called vital phenomena 

 which take place in the textures of organisms are, for the most 

 part, performed by the agency of the living cell elements, in 

 which we can recognize independent manifestations of life, such 

 as the response to stimuli, motion, nutrition, growth, etc. The 

 living activity of organisms requires for its perfect development 

 certain external conditions, namely, a certain degree of warmth 

 and moisture. Without heat and moisture the chemical inter- 

 changes 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 

 followed by death. 



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

 which their size, form and functional activity are constantly un- 

 dergoing some general alteration dependent on, or concurrent 

 with, 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 remarkable 

 faculty of producing individuals like themselves. 



This is accomplished by setting apart a cell which, under 

 favorable circumstances, assumes special powers of growth, in- 

 creases in size by the rapid formation of new cells, and develops 

 into an independent living unit. In time it arrives at maturity, 

 and becomes like its parent, and then passes through the same 

 cycle by its power of assimilation it grows to maturity, repro- 

 duces its like, decays and dies. 



