THE CONTROL OF RESPIRATION 83 



Nowadays the medullary respiratory centre is not regarded 

 as being composed of distinct inspiratory and expiratory centres 

 although there are certainly neurones which are more active 

 during one or other of these two phases. These units are distri- 

 buted throughout the medullary region, however, and inter- 

 mingle with one another. The evidence for the two centres 

 derived from stimulation experiments is difficult to interpret 

 because of the problem of localising the stimulus, which is also 

 unnatural because it produces a synchronous discharge of many 

 neurones. 



It may be concluded that the genesis of the respiratory rhythm 

 is due to the interaction between the medullary neurones and 

 the interplay of descending impulses from the pontine regions 

 but may require some non-specific afferent input for its 

 continuous expression. This accords well with the general 

 views derived from recent studies on motor rhythmicity of 

 other types, e.g., the swimmeret rhythm of crayfish. 



(b) CHEMICAL REGULATION OF RESPIRATION 



It is known from the classical works of Haldane that the COg 

 content of the blood and alveolar air plays a dominant role in 

 the regulation of respiration in mammals. Many mechanisms 

 interact to maintain the composition of the alveolar air remark- 

 ably constant over quite a wide range of conditions. Minute 

 increases in the CO2 content of the alveolar air (e.g., 0-25%) 

 lead to a very marked increase (100%) in the volume of air 

 breathed. Lowering of the alveolar CO 2 by breathing as deeply 

 and rapidly as possible results in a cessation of breathing and a 

 return of the CO 2 tension to its normal level. The ability to hold 

 one's breath, e.g., during diving, is increased by such a pre- 

 liminary hyperventilation. Above certain concentrations (10% 

 of inspired air) the effects of CO 2 become harmful, anaesthetic, 

 and finally fatal above 40%. During resuscitation of a drowned 

 or asphyxiated person a mixture of 95% O2 and 5% CO2 was 

 formerly used. Nowadays the CO 2 is omitted because it would 

 probably have a tension less than that already in the blood, 



