FACTORS INFLUENCING RATE OF METABOLISM 49 



fall in blood pressure; if the thalamus is also removed 

 there is a further loss of ability to regulate body tem- 

 perature (Rogers, 1920). The nervous control of me- 

 tabolism and body temperature may be exercised in indi- 

 rect ways — such as the stimulation of a gland which 

 gives off a secretion which in turn increases oxidation 

 or changes blood pressure. 



Rhythms. — ^All living things show more or less rhyth- 

 mical changes in metabolic rate. Such rhythm is one of 

 the fundamental properties of living substance: the vis- 

 cosity of protoplasm hinders discharge of energy ; proto- 

 plasm is unstable and trigger action discharges energy 

 during katabolism ; endothermic anabolic stages are con- 

 cerned with reconstruction; katabolism and anabolism 

 must alternate more or less. Periodicity is in part a 

 consequence of the colloidal state (Sager, 1923). Ani- 

 mals are generally adjusted to rhythms. These are 

 characterized by the alternation of periods of rest and 

 activity. They are no more characteristic of poikilo- 

 therms than of homoiotherms, except that metabolism in 

 the former is more directly under the control of the en- 

 vironment. 



Conclusions. — ^Poikilotherms do not appear to differ 

 essentially from homoiotherms in the methods employed 

 to carry on metabolic processes. The former show 

 greater variations in metabolic rate because they are 

 more influenced by variations in environment. The lat- 

 ter are more independent of environment because they 

 have controlling mechanisms which permit them to live 

 at a uniform rate if sufficient food is available to supply 

 energy. 



