284 RESPIRATION 



to concentrate the available blood flow towards the brain. In mus- 

 cular exertion there is also a rise of blood pressure, due partly to 

 the effect on the vasomotor center of excess of CO 2 and deficiency 

 of oxygen in the arterial blood, but perhaps partly also to a gen- 

 eral pressor action complementary to a local depressor action on 

 the arteries and veins concerned in supplying the muscles with 

 blood. 



We may compare the action of the bulbar centers controlling 

 blood pressure and heart rate with that of the respiratory center 

 in its linked responses to direct chemical and peripheral nervous 

 stimuli; but data are not yet available for carrying the com- 

 parison into detail. 



From this general survey of the experimental evidence relating 

 to the regulation of the circulation, it will be seen that the deciding 

 factor in determining the rate of circulation ancl local distribution 

 of blood flow is local or general deficiency or excess in the diffusion 

 pressures of the substances which enter into tissue metabolism, 

 and particularly deficiency or excess in the diffusion pressures of 

 oxygen and carbonic acid. Temperature is also a factor, but per- 

 haps not a different one, since the diffusion pressure of a sub- 

 stance varies as its absolute temperature. 



The regulation of the circulation may be abnormal in various 

 ways, and the present chapter would be incomplete without some 

 reference to this subject. The abnormality may arise from disease 

 or congenital defect of the heart or from operative interference, 

 but is very commonly due to disorder of the nervous regulation, 

 whether or not any organic defect is also present. Another form 

 of abnormal circulation is due to a deficient volume of blood, or to 

 abnormality in its composition. In all these cases the abnormal 

 circulation is reflected in abnormal breathing. Owing to the ab- 

 sence of adequate clinical or experimental investigations it is 

 difficult as yet to deal with this subject in a satisfactory manner, 

 and I can only attempt to discuss it tentatively in the light of what 

 is already known. 



The effect may first be considered of a valvular defect which 

 either causes narrowing of valvular openings (stenosis) or makes 

 a valve incompetent so that there is regurgitation. The effect of 

 this is that, other things being equal, more work is thrown on one 

 or another part of the heart. If this extra work is not serious it 

 may be completely met, and partly by a true hypertrophy of the 

 muscular substance on which the increased work is thrown; but 

 if the extra work is serious the action of the heart as a pump will 



