THE CONTROL OF RESPIRATION 85 



fluid. The carotid glomi have a high rate of oxygen usage and 

 it is probable that the accumulation of anaerobic metabolites 

 during Og lack is the effective stimulus. The carotid and aortic 

 bodies are relatively insensitive to COg which is mainly detected 

 by the medullary centres but they play a very important role 

 when the Og tension of the blood is reduced, e.g., when the 

 inspired air contains less than 14% O^. 



The sensitivity to CO 2 of the respiratory centre of aquatic 

 mammals is less than that of terrestrial forms. In this way they 

 are able to dive and hold their breaths for longer periods. In 

 fish the respiratory centre seems to have properties similar to 

 that of mammals, and among forms living in well-aerated con- 

 ditions the sensitivity to CO 2 may be greater than that of forms 

 living in swamps where its tension may be high. Adaptations to 

 an increased CO 2 tension in the blood seem to be a necessary 

 prerequisite for the coming on to land. Most fish so far in- 

 vestigated show an increased ventilation volume in response 

 to both decreased O2 and increased CO 2 tensions. The responses 

 increasing ventilation in environments containing high levels 

 of CO 2 but normal levels of O2 appear inappropriate, since they 

 will not apparently lead to an increased oxygenation of the 

 blood. Here we see one fundamental difference between a 

 gill and a lung which arises from the greater difficulty of 

 monitoring the CO 2 tension in the water passing away from the 

 gills. In a lung the CO 2 persists in the alveoli and serves to govern 

 the whole regulatory mechanism. Oxygen tension affects both 

 fish and mammalian regulation chiefly through receptors placed 

 in the aortic arches but also possibly in the gills of fishes. Cutting 

 the 9th and 10th nerves which innervate the gill epitheha pro- 

 duced a decreased response to reduced O2 tensions in tench. The 

 whole question of regulation of respiration in fishes has not 

 received the attention accorded to mammalian breathing and 

 remains in a much vaguer state. Nevertheless the general 

 organisation is similar, i.e., a meduUary respiratory centre of 

 distributed expiratory and inspiratory neurones able to generate 

 the rhythmic activity and which are influenced by CO2 tensions 

 in the blood. 



