1096 PHYSIOLOGY 



of the lungs on the respiratory centre, led Head to ascribe it to the summation 

 of a series of inhibitory stimuli. In these experiments, however, the fact 

 was forgotten that forced ventilation of the lungs with air or any inert gases 

 will reduce the carbon dioxide tension in the blood circulating round the 

 pulmonary alveoli and therefore round the respiratory centre. A respiratory 

 pause will therefore ensue and last until the increasing accumulation of 

 carbon dioxide in the blood raises its tension to the normal height, at which 

 the respiratory centre is ' set,' so to speak, to respond by a respiratory dis- 

 charge. If the carbon dioxide content of inspired air be increased to about 

 4-5 per cent., it is impossible to produce an apnceic pause, however rapidly 

 the respiratory movements be carried out. It would seem therefore that 

 ordinary apncea is entirely due to deficiency of carbon dioxide tension in the 

 respiratory centre, and that although the vagus nerve is inhibitory of 

 respiration, it is impossible to summate a series of vagus inhibitions by 

 artificial respiration so as to produce a lasting cessation of respiratory 

 movements. The chi ef use of the vagi in respiration seems to be for maintain- 

 ing, by frequent inhibitions, the excitability of the respiratory centre at a 

 maximum. 



M escher distinguished three types of apncea, viz. : 



Apncea vera, due to the washing out of CO 2 from the lungs, and the consequent re- 

 duction of the tension of this gas in the blood. 



Apncea vagi, a stoppage of respiration caused by stimulation of the inhibitory fibres 

 of the vagi. This stoppage is limited, as we have seen, to the immediate duration of 

 the stimulus (whether electric or produced by distension of the lungs). 



Apno&a spuria. Stoppage of respiration by stimulation of other nervous or sensory 

 surfaces. Thus when a duck plunges there is immediate stoppage of respiration, 

 which may last four or five minutes if the animal remains so long under water. The 

 same stoppage may be produced by pouring water on the beak. 



' CHEYNE-STOKES ' BREATHING 



If a man desires to hold his breath for some time he takes first a series 

 of deep breaths. The result is to diminish the carbon dioxide tension in the 

 alveoli and therefore to take away the need and the desire to breathe until 

 the carbon dioxide tension rises to normal as the result of the continued 

 formation of carbon dioxide. By continuing forced respiratory movements 

 for a minute or two the carbon dioxide tension both in the alveoli and in 

 the blood may be brought down to a very considerable extent. As a result 

 there is a prolonged period of apncea. During this period of cessation of 

 respirations, however, the oxygen is being used up, and the tension of this 

 gas in the alveoli may fall to such an extent that the respiratory centre is 

 excited by lack of oxygen before the carbon dioxide tension in the alveoli 

 has risen to its normal value. As a result of the excitation by oxygen lack, a 

 few breaths are taken and the carbon dioxide tension is once more lowered, 

 and the stimulation due to the oxygen lack, disappears. There is therefore 

 again a cessation of respiration. These periods of cessation alternate with 

 periods of respiration so that we get a condition of periodic breathing 



