384 PHYSIOLOGY. 



composition of the foods ingested. \Vith the carbohydrates the 

 quotient approaches unity. The carbohydrates contain in their 

 molecules enough oxygen to oxidize their hydrogen; all that remains 

 for the inspired oxygen is to burn up the carbon. The fats and 

 albumins, on the contrary, possess too little oxygen to burn all of 

 the hydrogen and nitrogen they contain. Hence all of the oxygen 

 is not found in the C0 2 eliminated, and the respiratory quotient falls 

 to 0.75. On a mixed diet the quotient is intermediate between 0.9 

 and 0.75. In plants the respiratory quotient, especially in starchy 

 ones, is equal to 1.0. In fatty seeds the respiratory quotient is. 

 0.6 to 0.8. 



Rate and Depth of Breathing on Respiration. Pfliiger has shown 

 that the increased rate of respiration does not cause a greater exchange 



Carbohydrates. 



Fig. 142. Variations of Respiratory Quotient According to the 

 Food Taken. (LANGLOIS.) 



of C0 2 and than slow breathing, excluding the effect of increased 

 activity of the muscles of respiration. 



Haldane and Priestly have shown that increasing the rate of 

 respiration diminishes its depth; hence the percentage of C0 2 in 

 alveolar air remains unaffected by the frequency. 



Effect of External Temperature. In the cold-blooded animal 

 the temperature and the respiratory exchange vary with the external 

 temperature. Warm-blooded animals in external cold increase and in 

 external warmth decrease their respiratory exchange and the produc- 

 tion of heat. They are greatly dependent upon the effect of the 

 nervous system upon the tone of the muscles. 



The response of warm-blooded animals to changes in temperature 

 can be abolished by curare or section of the spinal cord. Here the 

 muscles are not in tonus by the central nervous system. 



Very high external temperature will increase respiratory exchange, 



