THE EFFECT OF EXTERNAL INFLUENCES ON CELLS 59 



on the excitability, and the phenomena just mentioned are regarded by some 

 as the expression of actual stimulation. 



As an example of the directive * influence of heat on the movements of 

 cells (thermotaxis) the behavior of the ciliate Infusorian, Paramcecium, may 

 be mentioned. If the vessel in which they are contained is warmed on one 

 side to about 24-28 C. the animalcules withdraw to the other side, while 

 with a temperature below this limit they wander to the warmer side opposite 

 movements therefore according as the same stimulus is strong or weak. 



G. ELECTRICAL STIMULATION 



Because they are the most easily manipulated and most easily graduated, 

 electrical stimuli have been studied with very great exactness. Since their 

 effects are investigated chiefly on nerves and muscles of the vertebrates, we 

 shall deal with them at some length in presenting the physiology of nerves 

 and muscles. Suffice it for the present to state that it has been found in 

 nerves and muscles that the electrical current stimulates only at one pole or 

 the other, at the negative pole on closing the current, and at the positive on 

 breaking it (Pfliiger). Between the two poles it acts to change the excita- 

 bility, but not to stimulate. 



In Paramcecium, excitation is said to take place with the closing of the 

 current at the anode (Verworn). And there are other exceptions to the law 

 as it applies to nerve and muscle. Carlgren has shown with regard to Paramce-' 

 cium, that lifeless individuals, immediately after the closing of a sufficiently 

 strong constant current, show at the anode a shrinking up and at the cathode 

 bending movements, both of which are the consequence of the so-called cata- 

 phoric effect of the constant current [i. e., the tendency which this current has 

 to sweep substances in solution along with it ED.] ; and it is not at all improb- 

 able that similar phenomena in the living Paramcecium are of the same origin. 



The following phenomenon might be presented as a secondary effect of 

 the electric current. If a salamander (Amblystoma) be traversed longitu- 

 dinally by an electric current, the skin glands of the animal begin to produce 

 a copious secretion, which appears only on the side of the anode. The same 

 occurs also with isolated pieces of the animal in which the spinal cord has 

 been destroyed. But this secretion, as Loeb has shown, is not excited by the 

 current itself, but by the electro-positive ions liberated by the current. For 

 if the animal is immersed in a NaCl solution, the electro-positive ions in their 

 migration toward the cathode are set free on the skin of the animal and are 

 combined with the hydroxyl of water into NaOH. As direct experiments have 

 proved, this alkali exercises a powerful stimulating effect on the skin glands; 



1 Jennings is of the opinion that these so-called "directive influences " of light and of 

 heat are merely other instances of the selection by random movements of conditions favor- 

 able to the life processes. Taking Paramoecium as an example we find that when it wanders 

 into a degree of illumination or of temperature which is unfavorable, the organism is stim- 

 ulated by the change and reacts by making the usual motor response for avoiding an ob- 

 stacle. The total effect of many such responses is to carry the organism out of the field of 

 unfavorable influences or to keep it in the field of favorable ones. ED. 

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