Respiration and Metabolism 257 



This general account ot respiratory control in mammals pertains to both 

 eupneic and abnormal breathing. Certain mammals, however, show profound 

 adaptive differences in their respiratory patterns. These are the relatively 

 poikilothermic "hibernants" (bat, hamster, bear, marmot, etc), whose respira- 

 tory eccentricities are discussed in Chapter 10, and certain lower mammals, lor 

 example the armadillo (Edentata), capable of long periods of apnea despite 

 oxygen-dehciency and high blood carbon dioxide.-'-^ Diving birds and mam- 

 mals also have developed interesting adaptive mechanisms to permit apnea for 

 extended periods while submerged (see below). 



A good deal of work has accumulated to indicate well developed respiratory 

 control among fish. ^- ^'^^- --^- ^'^ The skate (Selachii) has a well deiined and 

 relatively autonomous respiratory center in the medulla. This center, which 

 may be surgically isolated anteriorly and posteriorly without impairing its 

 function, may be regarded as a mechanism functionally intermediate between 

 the segmentally arranged ganglia of the invertebrates and the compact and 

 complex specialized respiratory centers of the higher vertebrates."^ 



Certain tropical fish have been extensively studied in their responses to 

 respiratory stimulants.'"^ The electric eel. Electro phorus, a creek-dweller 

 capable of breathing air, is stimulated by oxygen deficiency, carbon dioxide 

 increase, and low pH. The yarrow, Erythrinus, a swamp-dweller in water of 

 about pH 4, can live in either water or air with its gill chamber closed off by 

 the operculum. Its respiratory control responds to oxygen lack, carbon dioxide 

 increase, and blood pH decrease down to 6. It relies on gills and aquatic 

 respiration in water containing 1.3-5 cc. O2/I. and 8-30 cc. CO2/I. At lower 

 oxygen tensions and at very low or very high carbon dioxide tensions, the 

 yarrow tends to become an air breather, taking air into the gas bladder (Fig. 

 58). 



In fish and amphibians the importance of respiratory center sensitivity to 

 carbon dioxide and its effect on respiratory movements is difficult to evaluate 

 because the COo tension does not ordinarily build up to any great degree, 

 owing to rapid diffusion of the gas through the integument and its solubility 

 in water. Further, considerable oxygen may be cutaneously derived, regardless 

 of breathing movements. In aquatic forms oxygen deficiency appears to be the 

 more profound respiratory stimulant; carbon dioxide is of less significance. 

 This situation is reversed in air breathers. 



In the frog, which is responsive to changes in both oxygen and carbon 

 dioxide, the "glandula carotica" may be considered analogous to the carotid 

 bodies of mammals. Carbon dioxide stimulates respiration when those glands 

 are either intact or obliterated (surgically or by painting with phenol). Oxygen 

 lack is effective, however, only when the glands are intact. •*•*- 



Reptilian respiration is influenced by the tension of the blood gases as well 

 as by temperature effects on the respiratory center cells themselves. ''-' -•^•' The 

 tortoise has both an inspiratory ("gasping") and an expiratory center. In the 

 diamond-back terrapin, Melaclemys, a high CO2 tolerance correlates with 

 diving capacity.-^" The reptilian respiratory mechanism may be associated 

 with swallowing and olfaction; in turtles and snakes the development of the 

 motor center of respiratory control parallels physiologically the esophageal 

 peristaltic center involved in swallowing reflexes.'^" The throat movements of 

 turtles, previously regarded as serving a respiratory pumping function, may be 



