PROTOPLASM AND COLLOIDS 



197 



isolated imisrle fibei-s and T hope my audi- 

 ence will ]iai'doii nie if I s])eak of this work 

 with somewhat more enthusiasm. 



A siujile muscle fiber is a rather pretty 

 preparation. The fibei-s I worked with 

 were dissected from the adductor magnus 

 muscle of the frog. In this I was fortunate 

 to have the help of Dr. E. W. Ashkenaz, 

 who is an expert at such dissection. The 

 fibers are cut to a couvenient length (they 

 may be an inch oi- more long). At the 

 cut ends a plug forms. The plug at the 

 cut end of a muscle fiber in all probability 

 represents a surfaee precipitatiou reactiou. 

 Appareutly the formatiou of the plug is 

 dependeut on calcium ion, but more work 

 needs to be done before this can be defi- 

 nitely established. 



If isolated muscle fibers with cut plugged 

 ends are placed in solutions of isosmotic 

 calcium chloride, an interesting phenom- 

 enon occurs, the fiber rapidly shortens. 

 So great is this shortening that after two 

 or three minutes the fiber is only a little 

 over a fourth of its original length. The 

 effect is due to an entrance of calcium into 

 the protoplasm of the fiber : if the fibers are 

 immersed only partially, so as not to ex- 

 pose their ends to calcium solution, no 

 shortening occurs; on the other hand, if 

 only the ends are exposed, the shortening 

 is as vigorous as usual. If one follows the 

 progress of the shortening under the micro- 

 scope, it can be seen that bit by bit the 

 protoplasm of this fiber is converted into 

 plug material (Pig. 5). Proceeding from 

 each end, the plugs grow inward, until the 

 entire fiber is transformed into a lifeless, 

 brownish-black mass of plug material. 

 This simple experiment shows two things. 

 In the first place, it provides a striking 

 demonstration of the fact that calcium 

 ion is capable of making the muscle proto- 

 plasm shorten to the very maximum of 

 shortening. And in the second place, it also 

 indicates that this shortening reaction is a 

 species of clotting or surface precipitation 

 reaction. For as the muscle protoplasm be- 

 comes shorter, it is directly converted into 

 a plug, and it has already been urged that 

 the formation of the plug is a species of 

 surface preciiDitation reaction. 



We see, therefore, that the isolated mus- 

 cle-fiber experiment offers support for the 

 second half of our theory of stimulation. 

 The protoplasm of muscle cells is extraordi- 

 narily and i:)eculiarly sensitive to calcium. 

 Aside from barium and strontium, other 

 cations do not act in this way. Let me take 

 time for a minute to consider the effect of 

 magnesium ion on the isolated nuiscle fiber. 

 You may remember that in the case of the 

 surface precipitation reaction in the sea- 

 urchin egg, magnesium acted like calcium, 

 but in far weaker fashion. So, too, in the 

 case of the muscle cell. Calcium can act in 

 dilution. Thus, for example, solutions of 

 calcium chloride one-half or even one- 

 fourth the strength of isosmotic solutions 

 cause rapid shortening of the muscle proto- 

 plasuL On the other hand, dilute solutions 

 of magnesium chloride cause no such effect. 

 Indeed, when a fiber is immersed in a solu- 

 tion of magnesium chloride of one-half 

 the isosmotic strength, the first noticeable 

 change is a lengthening of the fiber. How- 

 ever, in a solution five times the isosmotic 

 strength there is a rapid shortening. In 

 other words, if magnesium is sufficiently 

 concentrated it can act like calcium. This 

 is in accord with experiments on sea-urchin 



There is hardly time to consider other 

 observations on isolated muscle fibers. 



One point which I should perhaps men- 

 tion is that the fibers behave very much 

 like the Nereis eg:g in that their irritability 

 is to a large extent lost when they are im- 

 mersed in solutions which tend to remove 

 calcium. Thus this study of muscle fibers 

 lends support to the first half of the stimu- 

 lation theory, as well as to the second half. 



In this rather long talk, if I have done 

 nothing else, I believe I have shown you 

 the general similarity in the behavior of 

 various types of living cells. We have 

 skipped about from amebae to egg cells to 

 muscle fibers. In all these protoplasmic 

 systems, so different in external appear- 

 ance, the colloidal behavior of the proto- 

 plasm is essentially similar, and can, I be- 

 lieve, be interpreted in terms of our theory. 

 Protoplasm is sensitive to radiation, to the 

 electric current, to mechanical impact, be- 



