SUMMATION OF MECHANICAL RESPONSES. 379 



stimulus, but, as we have seen, the summit of the isotonic curve does 

 not usually correspond to the equilibrium length X. The question why 

 a muscle shortens to a less equilibrium length when it is tetanised, than 

 when it is maximally excited by a single stimulus, will be better 

 understood when we have had under consideration the order of the 

 changes which occur in response to a single stimulation. It will be 

 seen that the first of these changes is oxidation, of which the 

 excitatory electrical change is the sign and probably indicates the 

 duration, and that as soon as it has attained its full development it is 

 at once converted into mechanical tension (contractile stress). We 

 shall also see that, although the link which connects the chemical 

 change with the mechanical change is an unknown one, there is good 

 reason for believing that in whatever way mechanical potential energy 

 may arise out of chemical energy, no measurable time is lost in the 

 conversion. Its visible effect, on the other hand, i.e. the change of form, 

 is a process which requires time, and consequently overlasts its efficient 

 cause. At the moment that the change of form is beginning, the 

 electro-chemical change has already culminated. The mechanical effect 

 subsides the moment it has reached the degree called maximal, its 

 efficient cause being no longer operative. If, however, the excitation 

 is repeated before this subsidence begins, the effect of the second 

 excitation is added to that of the first, so that the curve may, as 

 Helmholtz's well-known experiment shows, attain nearly double the 

 height of the single isotonic curve. If a series of such excitations 

 follow one another at equal time-intervals, their effects are so summed 

 that the muscle shortens to a definite length, the difference between 

 which and A depends on the load. So that, when maximal excitations 

 are used, it is the summation of the effects of successive excitatory 

 changes, which causes the muscle to shorten to an equilibrium length 

 less than that attained by it in response to a single such excitation. 

 With reference to this summation, Fick's measurements (to be referred 

 to later), determining that in such a succession of excitations the 

 chemical changes as judged by their thermal effects are more intense 

 in the first than in the subsequent responses, must be borne in mind 

 in connection with the fact that, in the summation of effects, the incre- 

 ments due to the second and immediately following excitations are also 

 much larger than those due to succeeding ones. 



That the rise of the tetanus curve is due to something more than 

 summation of stimuli, i.e. in large measure to the summation of effects, 

 seems to be indicated by the fact that the electrical change, by which 

 the chemical one can best be made apparent, shows no sign of sum- 

 mation. The moment a muscle is tetanised, the so-called " action 

 current" has its full strength. Certain observations made by v. Prey, 1 

 relating to " supported " muscles, may have a similar bearing. He 

 alleges that if the load is so supported that the muscle does 

 not begin to lift it in response to a single excitation, until the 

 muscle has attained a length but little in excess of its length when 

 tetanised, it raises the load to the same height as that to which it 

 would be raised in tetanus ; the reason being that the amount of 

 shortening to be accomplished being less, the limited time during 

 which the muscular effort is at or near its maximum, has become 



1 v. Frey, " Versuche zur AuHosung der tetanischen Muskelcurve," Brltr. -.. Physiol. 

 Carl Lud wig, z. s. 70 Geburtst., Leipzig, 1886. 



