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IOWA ACADEMY OF SCIENCE Vor. XXIX, 1922 
production of work. Such suggestion has been made by several 
workers (Uexkull, Guenther, et al .) 3 on various grounds. In- 
deed it seems reasonable to believe as has been suggested by some, 
that there are two different contractile substances in muscle, one 
giving the usual quick “twitch,” the other the slower contraction, 
which exhibits itself as tone or contracture . 4 Assuming that there 
are two different contractile substances in the muscle it seems 
logical to infer that when the stimuli actively or excessively irri- 
tate the tonic substance at the same time with the activation ol 
the contractile substance, the successive responses would be ac- 
companied by gradual shortening, as indicated by the first type of 
curve, and on the other hand when the reverse condition prevails, 
that is to say when the stimuli affect slightly or not at all this 
peculiar tonic substance, the second type of curve would result. 
Doubtless between these two extremes, there are ranges of spe- 
cific irritability in each substance so that when compounded in 
various ratios under a given rate of stimulation and possibly 
under differences in metabolic condition prevailing in the muscle 
they would combine in responses to produce fatigue curves inter- 
mediate between the two that have been considered extremely 
typical. Indeed, during the course of this and previous investi- 
gations 5 such intermediate or gradation curves have not been 
lacking. Just what condition or conditions combine to. bring 
about relative proportional responses in the two substances, or 
what conditions must be fulfilled in order to predict just what 
type of fatigue curve will- result upon stimulation are as yet 
questions that need more light thrown upon them. In the light 
of recent investigations by Loeb, Osterhout and especially Lillie , 6 
it seems probable that muscular contraction is only one of the 
many phases of the general problems in physiology, such as irri- 
tability, conductility, stimulation, etc., awaiting solution, and like 
them is dependent in the ultimate analysis upon an understanding 
of fundamental physical and chemical changes in membrane per- 
meability. 
On referring to the simultaneous records B and B', which 
represent one of the extreme types of fatigue curves, a number of 
points are evident by comparison. Upon being stimulated once 
per second the muscle made in the course of seven minutes about 
3 Uexkall, Zentralblatt d. Physiologie, V. 22; p. 33, 1908, also Guenther, Am. Jr. 
Physiol., V. 14, p. 73, 1905. 
4 Iotekyo, Travaux du laboratoire de Physiologie, Institut Salvay, 5, 25. 
5 Baldwin, F. M., Am. Jr. Physiol., Vol. IyVI, pp. 127-139, 1921. 
6 Ivillie, Physiol. Rev., Vol. II, No. 2, p. 1, 1922. 
