August 23, 1912] 



SCIENCE 



251 



of a movement so rapid, responsive and 

 quickly reversible as muscular contraction, I 

 shall briefly recapitulate what I consider to 

 be the chief inadequacies of such hypotheses. 

 (1) The time required for a contraction 

 and relaxation due to swelling-changes in the 

 colloidal contractile elements — involving both 

 the incorporation of water into the colloid 

 and its release — seems to be too great. Pauli, 

 indeed, once estimated that if the elements 

 have the swelling properties of gelatine discs, 

 and the same surface-volume ratio as the 

 muscle-segments, the rate of swelling might 

 be sufficient to account for a contraction as 

 rapid as that of frogs' muscle. But the single 

 twitch may be many times shorter than this, 

 as in insects' wing-muscle, where moreover 

 the muscle-segments are larger than in verte- 

 brate muscle. It is of course possible to reply 

 to this objection that the actual elements con- 

 cerned may be much smaller than the visible 

 muscle-segments. Still, in any case, since the 

 single-twitch curve of striated muscle is 

 symmetrical, the rate of the water-absorption 

 would have to correspond in its time-rela- 

 tions with that of the water-loss. These 

 curves, however, have been shown to follow 

 different courses in those cases which have 

 been actually investigated (by van Bem- 

 melen). (2) A more serious difficulty is that 

 • the temperature-coefficient of the mechanical 

 energy of contraction, according to Bern- 

 stein's investigations, is negative; i. e., within 

 a normal range — ^up to 30° — the lifting power 

 of the twitch is increased, with a given inten- 

 sity of stimulus, by lowering the temperature. 

 The temperature-coefficient of swelling proc- 

 esses is however positive and large, like that 

 of chemical reactions; that of surface ten- 

 sion, on the contrary, is negative. Bernstein's 

 experiments must be regarded as very signifi- 

 cant, since the temperature-coefficients of the 

 chemical changes concerned in contraction 

 are of course positive; the above result would 

 seem therefore quite unaccountable if a chem- 

 ical change, like the production of acid, pre- 

 ceded and formed the condition of the contrac- 

 tile change. These experiments thus appear 

 to indicate, first, that the process immediately 



conditioning the contraction is only indi- 

 rectly dependent on a chemical change," and, 

 second, since this process has a negative tem- 

 perature-coefficient (a distinctive peculiarity 

 of surface-tension), that a change of surface- 

 tension in the ultimate contractile elements is 

 the direct source of the energy of contraction. 

 (3) A further disadvantage of the " swelling- 

 hypotheses " — as contrasted with the surface- 

 tension hypothesis — is that they offer no sug- 

 gestion as to the nature of the connection 

 between the electrical variation accompany- 

 ing contraction and the actual contractile 

 process. In electrical stimulation a change in 

 the electrical polarization of the plasma-mem- 

 brane is almost certainly the essential or 

 critical event. If the membrane enclosing a 

 colloidal system, such as a muscle-cell, changes 

 its polarization, a corresponding change of 

 polarization must occur at all of the electric- 

 ally polarized surfaces within the system thus 

 enclosed,' including presumably the surfaces 

 of the colloidal particles composing the con- 

 tractile elements. Changes of electrical polar- 

 ization at surfaces are definitely known to 

 alter the surface-tension. On the other 

 hand, the relation assumed to exist between 

 the polarization-change of stimulation and 

 the swelling of the colloidal elements, accord- 

 ing to the hypothesis under consideration, 

 must be exceedingly indirect. A chemical 

 change must first be assumed to occur, pro- 

 ducing substances, supposedly acid, which in- 

 crease the swelling properties of the colloid. 

 There is nothing unreasonable in this view, 

 but it fails to suggest any explanation of the 

 close parallelism which exists between the 

 electrical and the contractile processes in ac- 

 tive muscle. The interdependence between 

 the two is undoubtedly intimate, and is read- 

 ily intelligible on the hypothesis just outlined 



^ Resembling in this respect the plant-movements 

 due to changes in the permeability of the plasma- 

 membranes of turgid cells (in Mimosa, Dionoea, 

 etc.). 



^ Since the electrical potential of the whole cell- 

 interior changes, and with it one of the factors in 

 the conditions of equilibrium of the electrical 

 double layers at the polarized surfaces within the 

 cell. 



