54 DYNAMICS OF LIVING MATTER 



transformed into mechanical energy; but they refrained from stating 

 how this transformation occurs. Engelmann tried to fill this gap.* 

 Striped muscle consists of alternating stripes of optically isotropic and 

 anisotropic substance. Engelmann observed that in the contraction 

 of the muscle the anisotropic substance increases in volume, while the 

 isotropic substance decreases. As the total volume of the muscle does 

 not change during the contraction, Engelmann concluded that part" 

 of the liquid of the isotropic substance diffused into the anisotropic 

 during contraction. He showed by experiments on violin strings 

 (made of catgut) that such a process of absorption can be produced by 

 heat. The viohn strings show the same double refraction as the aniso- 

 tropic stripes in the muscle. When a viohn string is suspended in 

 water, and the latter suddenly heated, the string contracts, and is able 

 to lift a weight in this contraction. This shortening of the string is 

 caused by an absorption of water by the string; and this imbibition is 

 caused by the increase in temperature. Like the contracting muscle, 

 the viohn string, in this case, becomes shorter and thicker. The process 

 is reversible as the string elongates again upon cooling. 



In the case of the muscle, Engelmann assumes that through the 

 stimulus which causes muscular contraction, heat is produced (through 

 the oxidation of carbohydrates); and that the increase in temperature 

 causes the anisotropic substance to absorb water from the isotropic 

 substance. This causes the change of form in the muscle — the thicken- 

 ing and shortening — by which it is able to lift a weight. 



For such a shortening of the violin string through heating, an increase 

 of about io° in the temperature of the water is necessary, while 

 the temperature of a frog's muscle during a single contractiori increases 

 only by o.ooi°. Engelmann points out, however, that the increase in 

 the temperature of the whole muscle does not indicate the rise in tem- 

 perature in individual spots in the muscle, which may be considerably 

 higher. The foci of combustion heat the whole mass of the muscle, 

 and we measure only the latter increase of temperature, which may 

 of course be quite small. Provided we grant this, it is necessary to 

 assume that the heat is sufficiently rapidly dissipated by conduction to 

 allow the rapid succession of relaxation and contraction of the muscle 

 in tetanus. We know that the muscles can contract and relax many 

 times a second, e.g. the muscles of the wings of insects contract and 

 relax more than a hundred times a second. I do not believe that the 

 process of dissipation of heat in liquids is rapid enough to make Engel- 

 mann's hypothesis probable, or even possible. It is, however, conceiv- 

 able that with a slight modification his hypothesis may be rendered free 



* Engelmann, Ucber den Ursprung dtr Muskelkraft, Leipzig, 1893. 



