986 



PHYSIOLOGY 



a short period of asphyxia is compared with that produced by oxygen lack, 

 by a surplus of carbon dioxide, and by the injection of lactic acid into the 

 circulation. There are certain minor details in these curves which are of 

 interest. When the oxygen of the lungs is rapidly washed out with a neutral 

 gas the asphyxial rise comes on about half a minute later than it would 

 with pure asphyxia. In the latter case it seems that the first rise is due 

 to the accumulation of carbon dioxide. The rise, however, under nitrogen 

 when it occurs is extremely abrupt, and the subsequent fall of blood-pressure, 

 i.e. the heart failure, is earlier in onset and more rapid than with ordinary 

 asphyxia. When excess of carbon dioxide is administered, i.e. 5 to 10 



B 



220- 



180- 



1100- 



on 



C0 2 12- 4 per cent 

 2 30 per cent 



-220 



FIG. 455. Asphyxial blood-pressure changes in curarised cat. A, inhalation of 

 C0 2 . B, injection of lactic acid. (MATHISON.) 



per cent., a marked rise of pressure occurs which, like that produced by 

 oxygen lack, is almost entirely conditioned by stimulation of the vaso-motor 

 centres and resulting constriction of the peripheral arterioles. If a loop of 

 intestine be placed in a plethysmograph, it will be seen that the rise of 

 pressure coincides with a shrinkage in volume of the intestine, pointing to 

 a vascular constriction (Fig. 456). The rise of blood-pressure due to the 

 vascular constriction may be maintained for a considerable period, e.g. ten to 

 fifteen minutes, and we do not get the rapid fall of pressure due to failure 

 of the heart that is observed in an ordinary asphyxia tracing. If partial 

 oxygen lack or abnormally increased tension of carbon dioxide be continued 

 for some time, a state of narcosis or paralysis finally ensues which affects not 

 only the higher centres but also those of the medulla, so that death may ensue 

 without convulsions or excessive rise of blood-pressure. 



Is there any common factor in the two conditions of oxygen lack and 

 carbon dioxide excess which may account for the similarity in their effects ? 

 It has been shown that whenever there is a deficiency of oxygen the metabo- 



