INFLUENCE OF MUSCULAR EXERTION UPON METABOLISM. 191 
found that when the motor nerves of the rabbit are paralyzed by 
curari the rate of metabolism, as measured by the respiratory ex- 
change, falls to about one half the amount during rest and does 
not react to changes of external temperature. Pfltiger * computes 
from his experiments a similar reduction of about 35 per cent. 
Under these conditions the heat production of an animal is insuffi- 
cient to maintain its normal temperature, and unless the loss of 
heat from the body is hindered by coverings or otherwise it soon 
perishes. Frank & F. Voit,; on the contrary, found that curarized 
dogs excreted no less carbon dioxide than in the normal state, pro- 
vided the body temperature was kept normal. 
Secondary Effects of Muscular Exertion. 
The greater activity of the muscular metabolism during the 
performance of work gives rise to important secondary effects, par- 
ticularly upon the circulation and respiration. It is a familiar fact 
that in active exercise the action of the heart is largely increased 
and the breathing becomes deeper and more rapid, and that ordi- 
narily the limit of muscular exertion is set, not by the power of the 
muscles themselves, but by the ability of the heart and lungs to 
keep pace with the demands upon them. 
CrrcuLaAtTion.—The circulating blood is the medium by which 
oxygen is conveyed to the muscles and carbon dioxide and other 
products of their metabolism removed. The latter function is of 
special importance, since an accumulation in the muscle of the 
products of its own metabolism speedily reduces and ultimately 
* suspends its power to contract. In active muscular exercise, there- 
fore, an increase in the rate of circulation is essential to the con- 
tinued activity of the muscles. This increase appears to be brought 
about by the accumulation in the blood of the products of metab- 
olism, which act as a stimulus to the vaso-motor center. The 
result is a dilation of the peripheral blood-vessels, which is aided by 
the mechanical effects of muscular contraction. To offset this and 
prevent a fall of arterial blood pressure, the visceral capillaries are 
probably constricted, while the rapidity and strength of the heart- 
beats are largely increased. The rapidity of the circulation as a 
* Arch. ges. Physiol., 18, 247. + Zeit. f. Biol., 42, 349. 
Digitized by Microsoft® 
