6 RESPIRATION 



oxygen supply. Lavoisier himself and afterwards Regnault and 

 Reiset found that a warm-blooded animal breathing pure oxygen 

 consumes no more oxygen than an animal breathing ordinary 

 air; and subsequent investigations have shown that the oxygen 

 percentage in air has to be reduced very low before the oxy- 

 gen consumption is diminished. Pfliiger also found that oxidation 

 in the tissues is within wide limits independent of the rate of supply 

 of oxygen through the blood circulation. We are thus again face 

 to face with "physiological requirements." 



When temperature and heat production in the living body came 

 to be studied physiologically the first striking fact discovered 

 was that however much the external temperature might vary 

 within wide limits, the body temperature of warm-blooded ani- 

 mals remained practically the same during, health. Similarly, 

 although the heat production might be increased several times 

 by muscular exertion there was no material increase of body 

 temperature, and it became quite evident that the rise of tempera- 

 ture in fever is not due to increased heat production, but to dis- 

 turbance in the nervous regulation of heat discharge from the 

 body. Finally, when the influence of variations in external tem- 

 perature on heat production in the body was measured, it was 

 found by a succession of observers, including, besides Lavoisier, 12 

 Crawford in 1788, and Pfliiger and others in more recent times, 

 that, particularly in small animals, a lowering of external tem- 

 perature evokes through the influence of the nervous system a 

 rise in heat production, so that heat production becomes subservi- 

 ent to the maintenance of body temperature. This maintenance 

 is therefore one of the factors determining physiological energy 

 requirements. 



When we inquire what determines the energy requirements of 

 the body as a whole we ' nd that the results of investigation point 

 us towards a numbei of associated conditions which we can 

 identify one by one by observation or experiment, but which ordi- 

 narily occur in conjunction with one another, and on an average 

 remain very constant. Thus the activity of the nervous system in 

 determining various forms of muscular and glandular activity 

 constitutes one of the chief factors. But the activities of the 

 nervous system are themselves subject to control in the form of 

 what we call on the one hand "fatigue," or on the other "exuber- 

 ance of spirits," finding its expression in man in games and what 

 appear at first sight to be mere "luxus" activities of all kinds. 



u Pfluger, Pfluger's Archw, XII, p. 282, 1876. 



