THE EFFECT OF EXTERNAL INFLUENCES ON CELLS 53 



lated and ciliated cells sometimes in a very high degree. Nerves and muscles 

 of the Metazoa as well as the contractile fibers of the single-celled organisms 

 behave in a similar manner. As regards muscle Hering has shown that vari- 

 ous substances which for a long time were supposed to stimulate chemically, 

 in fact stimulate by closing the demarcation current of the muscle. Here 

 belong the so-called physiological salt solution (0.6 per cent), solutions of 

 fixed alkalies up to 0.1 per cent, and different salt solutions. Solutions also, 

 which stimulate chemically, may cause muscular contractions in this way, if 

 they are good conductors. A purely chemical stimulation of the muscle takes 

 place therefore only by means of fluids which do not conduct electricity or 

 do so very poorly, or by means of substances applied only to the uninjured 

 longitudinal surface of the muscle. 



As Biedermann has shown, the cross-striated muscles of the frog fall into 

 rhythmical contractions, if they are placed in weak solutions of Na^HPO^ 

 Na,CO 3 , Na 2 SO 4 , NaOH. Loeb has studied these contractions further in the 

 light of the dissociation theory, and has come to the conclusion that they are 

 produced only by certain ions (e.g., Na, Cl, Li, F, Br, I), but are impeded or 

 rendered impossible by others (e. g., Ca, K, Mg, Be, Sr, Co, Mn) the excita- 

 bility of the muscle not being changed in either case. Hydroxyl and hydrogen 

 ions hasten the appearance of the contractions without being able directly to 

 call them forth. Solutions which do not contain electrolytes produce no such 

 contractions. 



The theoretical significance of these and related facts cannot be discussed 

 here because we cannot yet adequately survey the field recently opened up by 

 these investigations. We may, however, expect from this quarter very valuable 

 results on the chemical relations of the living being in the near future. 



Chemical stimuli which have a higher osmotic tension than that of the struc- 

 tures to be stimulated, may exercise an exciting influence or may alter the 

 excitability by the extraction of water, as probably occurs in many cases with 

 the nerves. 



That this is not the only determining factor however, and that the peculiar 

 properties of the chemical substance exercise an essential influence, appears from 

 the fact that equimolecular solutions in general stimulate more powerfully the 

 higher the molecular weight (Griitzner). Thus sodium iodide for example 

 stimulates more powerfully than the bromide and chloride, whereas the osmotic 

 tensions of all these is equal. 



Besides these direct responses to stimuli and the alterations of excitability, 

 which we must pass over, certain substances exercise a very remarkable influ- 

 ence 1 on the movements of free-living cells ly attracting or repelling them. 

 These phenomena are designated by the term cliemotaxis and are described in 

 the one case as positive, in the other as negative. Different substances exer- 

 cise different influences on various cells, and the same substance in different 

 concentrations may produce different effects on the same organism. 



Some examples of chemotaxis may be cited here. Certain forms of Bac- 

 teria are attracted by oxygen, and in a microscopical preparation which contains 

 these Bacteria together with some alga cells one may observe how they gather 



1 First demonstrated by Engelmann on the Bacteria. 



