THE MECHANISM OF LIFE 565 



The above-developed idea of the mechanism of amoeboid 

 protoplasmic movement has at once the great advantage that, 

 though modified by special conditions in individual cases, its prin- 

 ciples may be applied to all other phenomena of contraction, 

 to protoplasmic streaming in plant-cells as well as to ciliary 

 and muscular movement. We will here select only the most com- 

 plicated case, the movement of cross-striated muscles. Since the 

 same processes go on in all the individual muscle-segments, we 

 will limit ourselves to consideration of the single segment. As 

 has already been seen, 1 the muscle-segment consists of two 

 different substances, the more solid, anisotropic substance lying in 

 the middle, and the isotropic substance lying at the two sides 

 of the latter (Fig. 271). The phenomena that are visible with 

 the microscope during a contraction resulting from stimulation, as 

 Engelmann 2 and others have established in detail, consists 

 essentially in the flowing of isotropic substance from both 

 sides into the anisotropic ; thus the latter substance increases 



FIG. 271. Muscle-segments. /, At rest, //, in contraction ; A, in ordinary, , in polarized light, 

 a, Anisotropic, i, isotropic disks. 



in volume and the disk becomes broader, while the length of the 

 whole segment decreases correspondingly. Hence the elementary 

 fundamental phenomenon in muscle-contraction is a mixing of 

 two substances which at rest lie beside one another ; constituents 

 of the isotropic, or more mobile substance, force their way into the 

 anisotropic, or fixed substance. In this process the fact, which 

 E. A. Schafer ('91, 1, 2, 3,) discovered, is noteworthy, namely, that 

 the anisotropic substance, which does not change its place, offers 

 the greatest possible surface to the entrance of the isotropic 

 substance by means of the system of tubes already mentioned, 3 

 so that the intermingling is able to take place very rapidly 

 (Fig. 272). During the explosive decomposition of the biogens, 

 either in the isotropic or the anisotropic substance, which latter 

 is regarded by Engelmann as the specially contractile element, 

 the chemical constitution of the biogen-molecules is so changed 

 that a molecular attraction arises between them and certain 



1 Of. p. 243. 2 Of. p. 245. 3 Of. p. 245. 



