No. 5S2] SIGNIFICANCE OF INTERNAL CONDITIONS 355 



rones. The persistence of the common property accounts 

 for the asphyxial convulsions of the spinal animals and 

 for the movements which are sometimes considered to be 

 respiratory movements and commonly attributed to so- 

 called respiratory centers. But whether we consider that 

 the cells of the respiratory group have gradually acquired 

 a lower threshold value for stimulation by carbon dioxide 

 than the other cells of the nervous system, or that the 

 cells of the respiratory group have simply retained the 

 the common excitability of protoplasm in general to car- 

 bon dioxide, and the remaining cells have undergone modi- 

 fications which have raised their threshold value, makes 

 little difference from the theoretical point of view. In 

 an environment which is, so far as one can determine, 

 uniform, certain quantitative variations have occurred 

 which have resulted in a differential sensitiveness to car- 

 bon dioxide or to lack of oxygen. The changes have not 

 been qualitative, since asphyxial convulsions involving 

 muscles innervated from other parts of the central nerv- 

 ous system, may be brought on by a reduction of the 

 oxygen supply. 



The usefulness of the lower threshold value for lack 

 of oxygen in a particular group of cells is at once ap- 

 parent. Oxygen generation of the blood and elimination 

 of carbon dioxide proceed without attention, and without 

 noticeable excitation of any other group of nerve cells. 

 There is no disturbance of the precision of movement of 

 any group of muscles aside from those actually engaged 

 in the respiratory act, nor the slightest effect upon the 

 neurones involved in mental processes, resulting from the 

 decreased oxygen or increased carbon dioxide tension in 

 the blood sufficient to provoke a respiratory response. 



3. Stimulation in Terms of Chemical Equilibrium 

 This brings us to a consideration of the nature of stimu- 

 lation in general. Lack of space precludes all but the 

 briefest mention at this time. We may here simply in- 

 dicate the consideration in terms of the laws of mass 



