CHAPTER XIII 

 CHEMICAL REGULATION OF CELL ACTIVITIES 



IN previous chapters we have dealt with various aspects 

 of the chemical activities of living organisms. We have 

 treated these activities from the chemical aspect, but it 

 is well known that the activities of cells vary from time to 

 time and that they are influenced by outside agencies. 



In the animal kingdom a specialised system has been built 

 up by which the various tissues are regulated and controlled 

 so that they all work together for the good of the organism. 

 How the nervous system performs this function is unknown, 

 but it is a physico-chemical problem which must be solved 

 in the future. We know that by means of nerves various 

 activities can be augmented or diminished, and that when such 

 a result is produced an electrical change occurs in the conduct- 

 ing nerve : it is probably related in some way to a setting 

 free of electrolytes. 



Stimulation of sensory nerve endings is brought about by 

 photochemical changes, or by some shift in equilibrium 

 produced by pressure, temperature, or other physical change, 

 similar to the changes shown by the dilatometer, where 

 volume changes are registered during allotropic transforma- 

 tions, or to the changes in allotropic form which occur at 

 certain transition temperatures. 



The development of a conducting system of nerves gives 

 rise to a special form of tissue. The conducting portions of 

 this tissue differ somewhat from the cellular parts where the 

 nerve cells are situated, and from the chemical point of view 

 the difference in composition between executive structure, 

 such as muscle or glands, and controlling and conducting 

 tissues, is of interest. 



In addition to the regulation by nerve impulses there is a 

 regulation by means of chemical substances which in the higher 

 vertebrates are transported by the circulating blood. This 

 is the modern counterpart of the early "humoral" conceptions. 



We have seen several examples of this regulation in preced- 

 ing chapters. The accessory food substances, secretin and 

 gastrin, are examples of these regulators. 



Waste products from the tissues act on other organs. 

 Carbon dioxide, which is produced in increased amount by 



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