258 



Comparative Animal Physiology 



regarded as "sniffing" reflexes, and are in reality related to food stimulation and 

 olfaction.-*-' The breathing cycle in the tortoise includes a pronounced com- 

 pression phase, frequently followed by an apneic interval. Of further interest 

 is the variability in intratracheal pressure developed from respiratory move- 

 ments with the glottis closed except during actual inspiration and expiration. 



Oxygen deficiency serves as a respiratory stimulant in birds, but species 

 diff^erences are found in the response to carbon dioxide. ^^-^ Carbon dioxide 

 stimulates in the mallard duck, English sparrow, and starling, for instance, but 

 causes apnea in muscovy and pekin ducks. Evidence exists for COo inhibitory 

 reflexes originating in chemoreceptors in the nasopharynx. 



A review of breathing in vertebrates indicates that central respiratory centers 

 are present among fishes, amphibians, reptiles, birds, and mammals. The 

 development of this control has made possible responses to small changes 



TABLE 46. DURATION OF DIVES OF DIVING MAMMALS (AFTER IRVING^ 



in blood CO2. Chemoreceptors, analogous to carotid bodies, have been demon- 

 strated in Amphibia and in more advanced groups, and permit coordinated 

 responses to oxygen lack. Further work is needed to clarify the relative im- 

 portance of these two types of operating mechanisms, particularly among lower 

 vertebrates, as well as the role of hydrogen ions, other than those originating 

 from carbonic acid, as a respiratory stimulant. 



Respiratory Control in Diving Birds and Mammals. In diving birds and 

 mammals a number of adaptations in respiration, metabolism, blood, and 

 circulation combine to permit survival under water for long periods (Table 

 46). The major respiratory changes which make such diving po.ssible are not 



