186 PRINCIPLES OF ANIMAL NUTRITION. 
series of simple contractions which fuse together, resulting in a state 
of contraction which continues, subject to the effects of fatigue, as 
long as the stimulus acts. This form of muscular contraction 
has received the name of “tetanus.” In the living animal the 
ordinary contractions of the muscles brought about through the 
nervous system, even those that seem but momentary, are essen- 
tially tetanic in their character. 
Chemical Changes during Contraction.—Under the influence 
of a stimulus sufficient to produce a muscular contraction there 
occurs a sudden and large increase in the chemical changes which 
are continually going on even in the quiescent muscle. More mate- 
rial is metabolized in the muscle during contraction and energy is 
thus liberated for the performance of work. 
Our knowledge of the nature of these chemical changes in the 
contracting muscle is comparatively meager, but three main features 
appear well established: 
First, during contraction the neutral or slightly alkaline reac- 
tion of the quiescent muscle c’ anges to an acid reaction, probably 
through the formation of sarcolactic acid. 
Second, there is a large increase in the amount of oxygen taken 
up by the muscle from the blood and a still greater increase in the 
amount of carbon dioxide given off by it.* 
Third, under normal circumstances, judging from the amount 
ofthe urinary nitrogen, there appears to be no considerable increase 
in the nitrogenous products of metabolism. 
From the increase in oxygen consumed and carbon dioxide given 
off we might be led at first thought to suppose that the increased 
activity in the muscle during contraction was of the nature of a 
simple oxidation. Certain other facts, however, seem to show that 
this view of the matter is inadequate. 
Oxipations IncomPLETE.—That the increased metabolism in 
the contracting muscle is not a simple oxidation of some material 
t carbon dioxide and water is indicated by the fact of the produc- 
tion of lactic or other acid in the muscle. Plainly, if the energy for 
muscular contraction is produced by oxidation the oxidation is at 
least incomplete. 
* Some good authorities doubt whether the carbon dioxide resulting 
from muscular exertion actually leaves the muscle in that form. Compare 
Schiffer, Text-book of Physiology, 1898, Vol. I, p. 911. 
