STIMULI AND THEIR ACTIONS 365 



work permits one phase to come more to the front, whether it be 

 metabolism, or change of form, or transformation of energy, it is 

 advantageous to select for the study of any vital phenomenon a 

 specific form of cell in which the vital phenomenon in question is 

 expressed especially clearly. By this method the phenomena of 

 changes of substance, of form and of energy may be considered 

 separately in different objects. But this ought never to lead us 

 into considering these different groups of phenomena as mutually 

 independent. They are merely different phases of one and the 

 same process. 



A. THE ACTIONS OF THE VARIOUS STIMULI 



1. The Actions of Chemical Stimuli 



The number of chemical bodies that when brought into contact 

 with living substance enter into chemical relation with its con- 

 stituents is enormous, but thus far only a few of them have been 

 investigated as regards their stimulating effects. A comprehensive, 

 comparative, cell-physiological investigation of chemical stimuli 

 and their actions, undertaken from a systematic point of view, 

 would require a very long time, but would surely yield very 

 valuable results. For the present, our knowledge of these stimuli 

 and their effects is so full of gaps that a systematic summary of it 

 is not possible. We must, therefore, limit ourselves to the con- 

 sideration of a few typical phenomena. 



a. The Phenomena of Excitation 



In general, increase in the quantity of ingested food-stuffs acts 

 as a chemical stimulus to augment metabolism. The best example 

 is afforded by the cells of the various tissues of the human body, 

 the most essential food-stuff of which is proteid. As Voit ('81) 

 has shown, a strong man, working hard, needs 118 grs. of proteid 

 in order to maintain his nitrogenous equilibrium intact, i.e., in 

 order to replace the quantity of nitrogen derived from the 

 destruction of the living substance of his cells arid excreted in the 

 urine. If this quantity of ingested proteid, which is a necessary 

 vital condition, be increased, as is the case with most men living 

 under good conditions, the greater quantity is not employed for 

 the construction of new cells, for the increase of living substance, 

 but is taken up by the tissue-cells from the blood, passed over 

 into living proteid and split up, to leave the body again almost 

 completely in the urine as the products of retrogressive proteid- 

 metamorphosis (urea, uric acid, creatinin, etc.). The increase of 

 the proteid-income beyond a certain measure (118 grs.)accomplishes, 



