190 PRINCIPLES OF ANIMAL NUTRITION. 
Muscular Tonus.—The chemical and thermal changes just 
enumerated as characterizing the muscle during contraction are 
taking place in it to a less extent at all times. Even at rest the 
muscle respires and produces heat, as is well illustrated. by Sczel- 
kow’s and Chauveau & Kaufmann’s experiments quoted above. 
The living muscles of the: body are elastic and may be said to 
be always slightly on the stretch, as is shown by the fact that when 
cut they gape open and that they shorten when their attachments to 
the bones are severed. This slight degree of contraction of the resting 
muscles has been called muscular tonus, and it is at least a plausible 
conclusion that the chemical changes taking place in a quiescent 
muscle furnish the energy to maintain this tonus. According to 
Chauveau * we may regard the essence of muscular contraction as a 
sudden increase in the elasticity of the muscle. He holds that all 
the energy liberated by muscular metabolism is converted first into 
the elastic force of the muscle and only secondarily into heat. Ac- 
cording to this view the slight degree of elasticity of the quiescent 
muscle is produced by the constant metabolism going on within it. 
In active muscular contraction this process is greatly exaggerated 
and the katabolic processes exceed the anabolic, thus giving rise to 
a great increase in muscular elasticity which in turn may be con- 
verted into work. In repose following work, we may assume that 
the substances broken down during contraction are built up again, 
while in prolonged repose the two processes must substantially 
balance each other. 
Muscular tonus is most noticeable during the waking hours, 
under the influence of external stimuli to the central nervous sys- 
tem, and consequently the rate of metabolism and the heat produc- 
tion tend to be greater than during sleep. To this is to be added, as 
a further cause of greater metabolic activity during the waking hours, 
those continual slight movements of the body which usually take 
place even in what is commonly spoken of as a state of rest and 
which may be designated as incidental movements. 
That the total amount of metabolism required for the mainte- 
nance of muscular tonus is considerable seems to be indicated by 
the observations of Rohrig & Zuntz,t and of Colasanti,t who 
* Le Travail Musculaire et l’Energie qu'il Represente. Paris, 1891. 
+ Arch. ges. Physiol., 4, 57; 12, 522. ft Ibid., 16, 157. 
