INTERNAL WORK. 365 
ume, as Voit states. The curved surface of the cylinder will be 
proportional to the square root of its volume, while the surface of 
the two ends will be proportional to the volume, and the ratio of 
total surface to volume will depend upon the ratio of length to 
diameter, being greater as the latter becomes less. 
Obviously, the calculation of the surface of an animal from its 
weight is a more or less uncertain one, and it is not surprising that 
the results should be somewhat fluctuating. It seems very doubt- 
ful, however, whether the larger. differences found in Voit’s com- 
pilation can be explained in this way, and Voit shows that there 
is another factor to be considered, viz., the mass of active cells in 
the body, which has a material bearing on the results. Before 
proceeding to a discussion of this point, however, it is desirable to 
consider briefly the significance of the general fact of the close 
relation between heat production and surface. 
Significance of Results.—Let us imagine an animal exposed to 
its “critical thermal environment” (p. 358) to gradually shrink in 
size while the external conditions remain the same. Under such 
circumstances the loss of heat to its surroundings will tend to in- 
crease relatively to its mass—that is, the body, like an inanimate 
object, will tend to cool more rapidly. This tendency can be met 
and the body temperature maintained in only two ways, viz., either 
by some modification of its surface—e.g., thicker hair—which will 
lower what we may call its emission constant, or by a relative in- 
crease in its rate of heat production. 
The results which we have been considering show that in 
general the emission constant, ie. the rate of heat emission per 
unit of surface, is substantially the same in small and large animals, 
and that the greater loss of heat in the former case is met by an 
increased production. In this aspect the effect is simply an ex- 
tension of the influence of falling temperature, the increased de- 
mand for heat being met by an increased supply, so that the extent 
of surface appears as the determining factor of the amount of met- 
abolism. 
In the case of an animal exposed to a temperature below the 
critical point, however, the increased demand for heat appears to be 
met largely by a stimulation of those processes of metabolism which 
do not result in any visible form of work, while the internal work, 
