REGULATION OF ENVIRONMENT _ 81 
ered. The capillary walls seem to take up the liquid 
and solid material required, and this material is at the 
same time reconstituted so as to produce blood plasma 
of normal composition. But the regeneration of the 
lost red corpuscles is a much slower process, so that the 
new blood is at first very deficient in corpuscles, and 
several weeks may be needed for their complete regen- 
eration. If, however, the bleeding is repeated at inter- 
vals the process of regeneration of corpuscles becomes 
faster and faster, so that frequent re-bleedings can 
be easily borne by the animal. Similarly, if blood is 
transfused from one animal to another the liquid part 
of the injected blood is rapidly eliminated, but not the 
red corpuscles. Hence for a considerable time the 
blood is abnormally rich in corpuscles. If, however, 
the transfusion is several times repeated the excess of 
corpuscles disappears more and more rapidly. The 
capacity of both the blood-forming and the blood-de- 
stroying process is thus increased by use. Young red 
corpuscles are known to be formed in the bone-mar- 
row, while the products of destruction of red cor- 
puscles are found in the liver and excreted in the bile. 
The capacity for formation or destruction of corpus- 
cles is thus associated with the physiological activity 
of these parts of the body, but this activity is evidently 
regulated with great exactitude. 
If we look, not merely at the internal, but also at the 
external activities of an organism Claude Bernard’s 
generalisation seems still to hold. The co-ordinated 
activities of the senses and muscular system are mainly 
directed to the end of providing for nutrition. Behind 
