76 ORGANISM AND ENVIRONMENT 



the heart, or the nervous system, which is the regu- 

 lator, but the metabolic activity of the body as a whole. 

 The blood circulates at such a rate as is sufficient to 

 keep its composition approximately constant at any 

 part of the body, and the rate of flow seems to be 

 greater or less at any one part in proportion as the 

 causes tending to disturb the composition of the blood 

 are greater or less at the same part. Among the chief 

 of these causes is consumption of oxygen and libera- 

 tion of CO 2 . Hence the circulation rate is to a large 

 extent determined by the activity of the latter pro- 

 cesses, and varies, just as the breathing varies, in such 

 a way as to keep the gas pressures in each part of the 

 body approximately constant. 



This is not an isolated fact in physiology. Claude 

 Bernard pointed out in 1878 in his Legons sur les 

 phenomenes de la vie that the blood is a fluid of re- 

 markably constant composition, and practically pro- 

 vides a constant internal environment for the living 

 cells of which the body of a compound organism is 

 made up. He seems to have been led to this conclu- 

 sion by his well-known studies on the sugar of the 

 blood. While still under the influence of the old ideas 

 of the blood as a very variable liquid he began his 

 investigations under the expectation that the amount 

 of sugar in the blood would vary in proportion to the 

 sugar absorbed by the intestine, and would disappear 

 when no sugar or other food was taken. To his 

 astonishment, however, he found sugar still abun- 

 dantly present in the blood during starvation, and that 

 any increase which he could produce in the blood 



