STIMULI AND THEIR ACTIONS 413 



lular organisms. This fact is associated with the one-sided 

 development of physiology as the science of the organs of verte- 

 brates. If experiment be limited to the organs of the higher 

 animal body, where almost all tissues depend upon the nervous 

 system, stimulation of the tissues may be indirect, through the 

 nerves supplying them ; in most cases it is necessarily so, since the 

 extremely finely branched nerve-fibres lying between the tissue- 

 cells can be excluded with difficulty. Methods of excluding 

 completely the influence of the nervous system are known only 

 in the case of muscle, by means of the very remarkable arrow- 

 poison of the Mexican Indians, curare. The cells of gland-tissue, 

 mucous membranes, connective tissue, etc., cannot be freed from 

 the influence of the nerves supplying them, and hence, if an 

 electric current is allowed to act upon them, because of the 

 much greater irritability of the nerve-fibres there is always ob- 

 tained not a direct stimulation of the tissue-cells alone, but a 

 simultaneous stimulation of the nerve-fibres which transmit their 

 own excitation to the gland-cell, the connective tissue-cell, etc. 

 In order to put a tissue into activity by stimulation, it is of course 

 :sufficient, and it is very convenient, to stimulate it indirectly 

 through its nerves ; but the effect of a direct stimulation of the 

 tissue itself cannot thus be studied. It follows that all the in- 

 numerable experiments with electrical stimulation upon the 

 vertebrate body have to do almost exclusively with stimulation 

 of nerve or muscle. 



This fact has led to many erroneous ideas regarding the 

 excitation-effects of galvanic stimulation. When attention was 

 confined chiefly to the stimulation of muscle, whether directly or 

 indirectly through the nerve, it was customary to consider con- 

 traction as the expression of excitation in the muscle. This was 

 undoubtedly correct. But the more or less manifest thought was 

 incorrect, that excitation exists only where contraction appears, 

 and that no excitation is present where there is no contraction. 

 This view has led to very many errors, many of which are not yet 

 corrected. Thus, the idea is still maintained by many physiologists, 

 that only variations in intensity of the galvanic current act to 

 stimulate, and only these when they occur with a certain rapidity ; 

 i.e., that only an increase or diminution in the strength of the 

 current, taking place at a certain rate, produces reactions, and 

 not a current continued with a constant intensity, or one that is 

 very gradually increased or decreased. It was believed that this 

 conclusion could be drawn from the following facts. 



If a constant current be allowed to flow through a muscle or its 

 nerve, the muscle contracts only at the moment of making, when, 

 therefore, the intensity of the current suddenly increases; it 

 expands again immediately, remains extended during the whole 

 duration of the current, and at the moment of breaking, when the 



