254 Coinparative Animal Physiology 



by way oF a permanent adjustment to complete anoxia or through the tem- 

 porary expediency of accumulating lactic acid and an oxygen debt during 

 periods of oxygen deficiency— a problem further elaborated below. 



In conclusion, the response of oxygen consumption rate to changes in oxygen 

 tension depends on and is characteristic of the individual. Internal factors 

 such as blood pigment, tissue saturation, and basal metabolic rate affect this 

 relationship. Environmental effects are important, including temperature, pH, 

 and rate of change of the oxygen tension. Two general reaction patterns are 

 found: (1) that in which the oxygen consumption is highly dependent on 

 tension, the non-regulatory type, and (2) that in which consumption is 

 independent of oxygen pressure over a wide range, the regulatory type. Further 

 work is needed to clarify the interrelation of the factors operating in this regu- 

 lation. It would be of interest to determine, for example, the effects of respira- 

 tory inhibitors— carbon monoxide, cyanide, and azide— on the ability to regulate 

 at extremes of the consumption-tension curve. More information is needed, to 

 add to that obtained on fish, pertaining to the effect of pH on t,.. Finally, a 

 fruitful line of investigation would appear to be that of determining the degree 

 of oxygen saturation of those tissues which manifest wide regulatory pro- 

 ficiency. 



Efficiency of organisms in an adaptive sense requires a compromise between 

 great activity under optimal conditions and wide tolerance under extreme 

 conditions. Consider the active brook trout and the enduring lungfish— each is 

 efficient in his own way. 



REGULATION OF BREATHING 



Breathing Movements in Higher Vertebrates. The respiratory movements 

 concerned in the breathing of organisms often involve large-scale ventilation, 

 as the pumping of insects or the undulating gyrations of T^lhifex during times 

 of low oxygen stress. On the other hand, respiratory movements may consist 

 of small motions, such as the action of spiracular cells to occlude the tracheal 

 tubes in response, for example, to local accumulation of lactic acid. Ventilation 

 movements and respiratory control in the higher vertebrates have been ex- 

 tensively considered in the classic physiological literature and in a number of 

 recent reviews.^-*'- ^■^^- '•^-' ^^^' ^««' -«- ^63, -'sa. 284, 285, air,. 382 yj^g ^gjiy com- 

 parative literature was ably summarized by Babak in Winterstein's Handbuch 

 der vergleichende Physiologic.*^ 



The control of respiration represents a balance betueen neurologic and 

 chemoreceptive mechanisms acting both directly and reflexly on the respiratory 

 center and in turn on the breathing mechanism itself. Normal inspiration in 

 mammals is an active process stimulated by impulses sent out continuously 

 over vagal and phrenic pathways from the inspiratory subdivision of the 

 medullary respiratory center.-*^*' The ensuing inspiration results in barrages 

 of sensory impulses, originating in the stretch receptors of the lungs them- 

 selves,"" and being returned to the respiratory center to inhibit lurther inspira- 

 tion in accordance with the well established I lering-Breuer reHex. Exhalation 

 is largely a passive process in normal eupneic breathing and does not generally 

 call forth expiration-inhibiting or inspiration-stimulating impulses, as originally 

 put forth as part of the Hering-Breuer mechanism. -'^^ I lowever, further nerv- 

 ous reflex control of breathing is indicated in hyperpnea in both the inspiratory 



