80 ORGANISM AND ENVIRONMENT 
we the means of determining the innumerable other 
substances present in blood we should doubtless dis- 
cover a similar delicacy of regulation. 
All parts of the body seem to participate in this 
regulation. We have already seen how this is so in the 
case of breathing, circulation, and the activities of the 
kidneys and liver. Recent investigations reveal the 
same thing in connection with such organs as the 
thyroid, suprarenal and pituitary glands. The regu- 
lation of the blood temperature in warm-blooded ani- 
mals is one of the most striking instances. During 
muscular exertion the heat production in the body 
may- be increased six or eight fold, but the tempera- 
ture of the arterial blood is only increased by a quite 
insignificant amount, as increase in the skin circula- 
tion and in the evaporation of moisture from the body 
compensates for the increased production of heat; 
while if the external temperature is varied the effects 
on the body temperature are also compensated by 
changes in the skin circulation and evaporation, and 
by variations in the heat-production of the body. The 
regulation is through the central nervous system, and 
is exactly comparable to the respiratory regulation of 
the blood. 
The phenomena observed after bleeding or transfu- 
sion of blood are of great interest in this connection, 
and have recently been studied in some detail by 
Boycott and Douglas, using the new method available 
for determining the total haemoglobin and total blood 
volume in the body during life. After bleeding the 
total blood volume in the body is very rapidly recov- 
