1106 PHYSIOLOGY 



after the commencement of the asphyxia. Again, if the volume of 

 the kidney be recorded by means of the oncometer, the rise of general 

 blood pressure produced by asphyxia is seen to be accompanied by a 

 marked shrinking of the kidney, and this shrinking endures until the 

 animal dies, showing that the fall of blood pressure following the 

 rise is due, not to a giving way of the arterial resistance, but to failure 

 of the heart. 



Similar results are obtained when the vessels to the brain are 

 ligatured, or when the animal has to respire an indifferent gas free 



A B 



t Resp.off -100 



on 



-ISO 



-100 



Nitrogen 



FIG. 457. Blood pressure changes in a cut. A, after cessation of respiratory 

 movements. B, as a result of artificial respiration with nitrogen. (MATH is ON.) 



from oxygen, such as nitrogen (Fig. 457, B) or hydrogen. In the uncu- 

 rarised animal the rise of blood pressure is associated with increased 

 respiratory movements and finally with convulsive spasms which may 

 involve practically every muscle of the body. 



We have spoken above of the phenomena of asphyxia as being 

 due to the circulation of venous blood. There are, however, two 

 factors which may be concerned and which may influence the medullary 

 centres and the heart. When the renewal of the lung ventilation is 

 stopped by ligature of the trachea or by cessation of the respiratory 

 movements, the increasing venosity of the blood involves a diminished 

 percentage of oxygen and an increased percentage of carbon dioxide, 

 and when asphyxia is excited by cessation of the circulation through 

 the medullary centres, these centres may suffer at the same time from 

 lack of oxygen and from the accumulation of carbon dioxide. The 

 question arises whether one or both of these factors are concerned. It 



