NERVOUS CONTROL OF THE BLOOD-VESSELS 1107 

 is easy to investigate the action of each separately. A pure oxygen 

 lack may be brought about by allowing an animal to breathe some 

 inert gas, such as nitrogen or hydrogen, or in the curarised animal one 

 of these gases may be administered by the pump used for artificial 

 respiration. The effects of accumulation of carbon dioxide in the 

 blood and tissues may be produced by the administration of gaseous 

 mixtures containing excess of oxygen, i.e. 30 to 40 per cent., with 

 varying percentages of carbon dioxide. In the first case, the tension 

 of the carbon dioxide in the blood will be kept below normal ; in the 

 A B 



220- 



180- 



\ too- 



on 



C0 2 12 -4 per cent 

 2 30 per cent 



-220 



FLQ. 458. Asphysical blood pressure changes in curarised cut. A, inhalation of 

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



second case, the tension of oxygen in the blood will be kept above 

 normal. In order to obtain results uncomplicated by the influence of 

 anaesthetics, the experiments may be carried out in animals which 

 have been deprived of consciousness by destruction of the brain above 

 the superior corpora quadrigemina. At different times physiologists 

 have been inclined to ascribe the excitatory phenomenon of asphyxia 

 either to absence of oxygen or to excess of carbon dioxide. Mathison 

 has shown that both conditions may concur in the production of the 

 rise of blood pressure in asphyxia. In Figs. 457 and 458 the rise of 

 arterial pressure produced by 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. 



