' 



386 MUSCULAR ACTION 



If these suppositions are true, there remains to be worked out the ther- 

 mogenic metabolism and also the spatial relations of the heating particles 

 to the structural elements of the sarcostyles. (See Fig. 239.) On what 

 basis to adapt this working-hypothesis to smooth, "unstriated" muscle 

 is not apparent. Little or nothing is known about the finer structure 

 of the myoids and muscle-fibrils, and this ignorance makes application 

 of so elaborate a theory out of the question at present. 



On the, whole, then, the thermogenic theory of sarcomeric imbibition 

 cannot be said to be wholly satisfactory; but it is, perhaps, the best so 

 far devised. 



THE CHEMI-SURFACE-TENSION THEORY. The chemi-surface-tension 

 theory has been of late perhaps better championed by Verworn than by 

 others. This theory, as set forth especially by him, has already been 

 given briefly in the chapter on protoplasm, but purely from the point of 

 view of undifferentiated protoplasm. Surface-tension is the cause of 

 the tendency to surface-contraction characteristic of liquids. It is this 

 force, for example, which makes a drop of dew on a leaf spherical instead 

 of flat. It is a potential energy of cohesion, apparent y, between the 

 adjacent molecules at the bounds of a mass of liquid. 



Perhaps we could not do better justice to this theory of muscular con- 

 traction than to state it in the terms of Verworn: "During the explo- 

 sive decomposition of the biogens [protoplasmic units] either in the iso- 

 tropic or the anisotropic substance (which latter is regarded by Engel- 

 mann as the specially contractile element), the chemical constitution of 

 the biogen-molecules is so changed that a molecular attraction arises 

 between them and certain constituents of the other substance. As a 

 result of this, the surface-tension between the two disks (the sarcomeres) 

 must necessarily diminish (or even become zero), i. e., an intermingling, 

 a mutual penetration of the two substances must take place. In this 

 process the isotropic, as the more mobile, substance will necessarily 

 diffuse into the anisotropic, as the more fixed, i. e., the muscle-segment 

 will necessarily decrease in length and increase in breadth. There will 

 thus be in principle the same process as in swelling, except that, as 

 Engelmann assumes, there will be not a simple admission of water, but 

 a chemical swelling, in which along with the water other chemical 

 substances will enter, such as take part in the regeneration of the decom- 

 posed biogen-molecules. But in proportion as these molecules are 

 regenerated and by the introduction of oxygen are brought back to the 

 maximum of their labile constitution, a change in the molecular rela- 

 tions occurs, and now, in contrast to what happened previously, a separa- 

 tion of the two substances will take place, which will give to the muscle- 

 segment its original form. 



"Although the processes, which for the present are wholly unknown, 

 may in reality take place very differently, at all events the principle of 

 modification of the molecular attraction by changes in the chemical 

 constitution of the molecules, the same principle that explains ameboid 

 movement, appears to be able to elucidate in its essential points the 



