REGULATION OF RESPIRATORY MOVEMENTS 1211 



of oxygen is insufficient to oxidise completely the materials consumed 

 in the production of the muscular energy. This acid mechanism is, 

 however, only employed when the supply of oxygen lags behind the 

 respiratory needs of the body (cp. Fig. 506). Ordinary exercise, even 

 when considerable, e.g. a twenty-four hours' track walking race, does 

 not cause, as Ryffel has shown, any appreciable increase in the elimina- 

 tion of lactic acid by the urine. Under normal circumstances the depth 

 and rhythm of respiration depend on the carbon dioxide pressure in 

 the respiratory centre, a rise of O2 per cent, of an atmosphere in the 

 tension of this gas in the alveoli being sufficient to double the amount 

 of alveolar ventilation during rest. 



The first phase in the phenomena of asphyxia is thus conditioned 



FIG. 506. Dissociation curve of oxyhaemoglobin in defibrinated cat's blood. 

 1, cat I, after partial occlusion of trachea and fifteen minutes breathing of 

 gas of increasing poverty in oxygen ; 4, cat II, at beginning of experiment ; 

 3, cat II, after fifteen minutes gas respiration ; 2, after twenty-one minutes 

 ditto. 



simply by the changes in the carbon dioxide tension. A little later 

 the gradual exhaustion of oxygen in the blood round the centre begins 

 to make itself felt. The respiratory centre shares with the rest of 

 the central nervous system a sensitiveness to the absence of oxygen, 

 deprivation of oxygen having first an excitatory and later a paralytic 

 effect. In asphyxia the first centres to feel this effect are those of 

 the cortex, and during the first stage there is mental excitation ter- 

 minating rapidly in abolition of consciousness. During the second 

 stage there is a discharge of energy, which spreads throughout the 

 whole nervous system, beginning in the bulbar centres and causing 

 a great rise of blood pressure with slowing of the heart, and extending 

 thence to all the spinal centres with the production of muscular 

 spasms. At this stage too there is a discharge of impulses giving 



