252 



SCIENCE 



[N. 8. Vol. XXXVI. No. 921 



that alterations of the surface-tension of the 

 contractile elements, conditioned by varia- 

 tions in their electrical surface-polarization, 

 form the immediate condition of contraction. 

 Any hypothesis which fails to explain this 

 interconnection must be regarded as seriously 

 deficient. 



I shall now discuss more fully what is 

 usually regarded as the alternative hypothesis, 

 namely, that the contractile energy is due to 

 changes in the surface-tension of certain 

 muscle-elements. In contraction the surface- 

 tension of these elements is supposedly in- 

 creased. If this increase of tension is suffi- 

 ciently great, and the area of the active 

 surface sufficiently large, the transformable 

 surface-energy, which is measured by the 

 product of these two factors, may be sufficient 

 to account for the work done by muscle in 

 contraction. The main question is whether 

 the evidence justifies us in assuming the ex- 

 istence of such conditions in the living mus- 

 cle-cell. Now the possible range of alteration 

 in the surface-tension of the colloidal con- 

 tractile elements under the influence of 

 changing electrical polarization is undoubt- 

 edly small — almost certainly less than ten 

 dynes per linear centimeter — so that in order 

 to develop the force observed in contraction a 

 very large active surface would be required. 

 Bernstein has shown that any possible altera- 

 tion of surface-tension at the surface of the 

 muscle-fibrils is quite insufficient to account 

 for the work done by contracting muscle; and 

 he has therefore assumed that in its ultimate 

 structure the fibril is built up out of smaller 

 ellipsoid contractile elements, by the altera- 

 tion of whose surface-tension the contraction 

 of the whole fibril is produced. By ascribing 

 sufficiently small dimensions to these elements 

 it is possible hypothetically to enlarge the 

 surface to the required degree. But is such 

 an assumption reconcilable with our present 

 knowledge of muscle structure? In Hiirthle's 

 photographs the fibril-segments show a sub- 

 division into narrower rodlet-like structures 

 ("Stabchen") which he regards as the ultimate 

 contractile elements. The united surface- 

 area even of these elements is also far from 



sufficient to meet the requirements. There are 

 thus no microscopically demonstrable structures 

 corresponding to Bernstein's hypothetical ellip- 

 soids. It is necessary, if the hypothesis is not 

 to be abandoned, to inquire if still smaller ele- 

 ments may not exist by the alteration of 

 whose surface-tension the requisite mechan- 

 ical energy may be produced. There is, as I 

 have pointed out elsewhere, good reason to re- 

 gard the ultimate colloidal particles of the 

 fibrils as corresponding to such elements. 

 By their union to form larger particles, as in 

 the general process of colloid-coagulation, 

 sufficient mechanical energy to account for 

 contraction might conceivably be freed, since 

 the reduction of surface-area in such a process 

 may be very great, implying a correspond- 

 ingly large transformation of surface-energy. 

 It is known that in the precipitation of col- 

 loids by electrolytes a fundamental condition 

 of the effect is a lessening of the contact-po- 

 tential of the particles against the medium; 

 the isoelectric point, where the contact-poten- 

 tial is zero, is typically the point of maximum 

 instability, i. e., the tendency to fusion of the 

 particles is then greatest; apparently their 

 surface-energy, then no longer compensated 

 by the electrical energy of the charged sur- 

 faces, draws the particles together until a 

 new condition of equilibrium with lessened 

 surface is reached. Increased surface-tension 

 resulting from decrease or disappearance of 

 the surface-charges is in fact now regarded 

 as one of the chief conditions determining the 

 union of the colloidal particles to form larger 

 complexes. This union, if it proceeds suffi- 

 ciently far, leads to the coagulation or pre- 

 cipitation of the colloid. 



Now if in a contractile tissue the mechan- 

 ical energy appearing in contraction is the ex- 

 pression of a temporary coalescence of the col- 

 loidal particles of the fibrils, due to their in- 

 creased surface-tension, any condition further- 

 ing contraction ought, if not rapidly reversed, 

 to lead to further fusion of particles and 

 eventually to visible coagulation. Various 

 facts of comparative physiology show that 

 during conditions of extreme contraction the 

 colloids of contractile tissues do frequently 



