216 NATURAL SCIENCE. . Marcu, 
It is interesting that Crawford and Lavoisier were the first to 
make experiments on the amount of heat given off by animals, and 
even to compare the oxidation, as represented chiefly by the carbonic 
acid, with the amount of heat produced. They showed that the 
oxidation could produce the necessary heat. 
John Davy in 18:5 showed by more exact experiments that there 
was no perceptible difference in the specific heat of arterial and of 
venous blood, and therefore Crawford’s theory must fall to the 
ground. This, however, by no means affected the doctrine of 
Lavoisier, that the oxidation in the animal body was the cause of the 
heat. In fact, Liebig not many years later, by properly interpreting 
the calorimetric experiments of Daling and Despretz, placed 
Lavoisier’s doctrine on such a firm basis that it became strong 
evidence for the Law of the Conservation of Energy. 
In the last fifty years the laboursof Helmholtz, Ludwig and Pfluger 
have proved that the heat is formedin the tissues, that heat is produced 
by the activity of muscles and glands. Bernard also, by showing 
how variations of the circulation of the bood could be effected by the 
nervous system, threw a great light upon the process of heat regula- 
tion. The formation of heat varied in different parts, but the tem- 
perature was equalised and regulated by the continual blood-stream ; 
the circulation of the blood, being under the control of the nervous 
system, could, when necessary, send more blood to the surface, and 
so increase the loss of heat, at another time could husband the 
warmth by diminishing the blood-supply to the skin. 
Such have been the chief lines along which the knowledge of the 
causes of animal temperature has advanced. The doctrine of ‘ vital 
heat,” propounded by the vitalistic school, has fallen before the 
advance of materialism; ‘‘ vital heat ’’’ is shown to be similar to heat 
arising from physical and chemical processes. 
Numerous observations upon animal temperature have now 
accumulated. Of these the most interesting, without doubt, les 
in the difference which has been found to exist between the 
lower and the higher animals. Those animals which are high 
in the scale of evolution, such as birds and mammals, have a 
high temperature, and this has a mean value (about 37° C. or 98° 
F. in man) which is independent of the temperature of the sur- 
rounding air. The lower animals on the contrary, such as molluscs, 
fishes, amphibians and reptiles, have a temperature generally only 
slightly above that of their surroundings. This difference between 
the two classes is expressed by the terms ‘“‘ warm-blooded” and 
‘““ cold-blooded ”’ animals. 
The warm-blooded animal has in correspondence with its high 
temperature a very energetic oxidation within its tissues, and this 
must, ceteris paribus, be more vigorous in the winter than in the 
summer. The cold-blooded animals, on the contrary, have a much 
smaller oxidation, and one which is so lessened by cold that in winter 
