290 A TEXTBOOK OF PHYSIOLOGY 



nervous connections have been cut, and cease when the circulation is 

 stopped, it is clear that the centre is dependent, in the first place, for 

 its activity upon the blood circulating through the centre. The 

 respiratory centre may be diagrammatically represented as follows 

 (Fig. lf,4): 



The question next arises as to what is the condition in the arterial 

 blood which calls the respiratory centre into action. Experimentally 

 it can be shown that blood which has been shaken up with s-omr 

 carbon dioxide gas causes, when injected into the peripheral end of the 

 vertebral or ca.otid artery that is, towards the brain an immediate 

 increase in the depth of the respiratory movements (Fig. 155). If 

 injected into the jugular vein, there may be no effect upon the 

 respiration; or if there be an effect, this will be delayed, and not by 



FIG. 155. ANAESTHETIZED EGG. 



Upper tracing, respiration ; lower tracing, blood-pressure. The arrow marks thd 

 injection of 20 c.c. of C0 2 and 2 saturated blood into peripheral carotid. 



any means marked. When injected in this manner, the excess of CO 2 

 may all be eliminated from the blood during its passage through 

 the lungs. If not all eliminated, it will, after fourteen or fifteen 

 seconds (the time of the lesser circulation), cause an effect upon res- 

 piration an effect which, as shown by the time of delay, is central in 

 origin (Fig. 156). The carbon dioxide of the blood, therefore, in some 

 way affects the respiratory movements centrally and not peripherally. 

 Again, if blood to which a little acid, such as lactic or butyric, 

 has been added be injected towards the brain, the respiratory move- 

 ments are likewise deepened. Thus, any increased acidity of the 

 blood affects the centre. It will be remembered that blood, as tested 

 by physical methods, is neutral. The H and HO ions balance one 

 another. Any excess of H ions at once stimulates the respiratory 

 centre. 



