NERVE 593 



In fact, it is doubtful whether any great reliance should be placed 

 on many of the observations hitherto made with this mode of excita- 

 tion. For it has been shown that the current of rest of the nerve 

 (p. 629), when a short-circuit is formed for it by a drop of any con- 

 ducting liquid applied to a fresh cross-section (the usual method of 

 experimenting on chemical stimulation), may of itself cause excitation 

 (Hering). 



Griitzner uses equimolecular solutions for experiments on chemical 

 stimulation i.e., solutions which contain an equal number of 

 molecules of the substances to be tested in a given volume of water. 

 He has found that for motor nerves the halogen salts have a stimu- 

 lating power in the order of their molecular weights ; e.g., sodium 

 iodide (Nal) is stronger than sodium bromide (NaBr), and sodium 

 bromide than sodium chloride (NaCI). Sensory nerves are much 

 less susceptible to chemical stimulation. Bile or bile salts, for 

 example, which stimulate motor nerves, have no effect on sensory.* 

 A sugar solution, which excites motor nerves, does not alter the rate 

 of respiration when applied to the central end of the vagus, which, 

 however, is excited by potassium chloride (p. 214). In non narcotized 

 animals reflex secretion of saliva is caused by stimulation of the 

 central end of the lingual with sodium chloride (Wertheimer). 



Mechanical stimulation has been carried to great perfection by 

 Heidenhain, and especially by Tigerstedt. By means of an instru- 

 ment invented by the latter, not only may a regular tetanus be 

 obtained, but the strength of the stimulus (fall of a weight) can be 

 graduated with fair accuracy within a considerable range. He found 

 that the smallest amount of work spent en a frog's nerve which would 

 suffice to excite it was a little less than a gramme-millimetre that is, 

 the work done by a gramme falling through a distance of a milli- 

 metre, or (taking an erg as equivalent to j^^ gramme-centimetre) 

 about 100 ergs. No doubt a great part of this is wasted, as a 

 much smaller quantity of work done by a beam of light on the 

 retina or by an electrical current on an isolated nerve, both of which 

 may be supposed to act more directly on the excitable constituents, 

 suffices to cause stimulation. Thus, the work done by the minimal, 

 natural or specific, stimulus for the retina in the form of green light 



may be as little as % erg (S. P. Langley), or only one-ten-thousand- 

 millionth part of the minimum work necessary for mechanical stimu- 

 lation. Again, with electrical stimulation (closure of a voltaic current, 

 or condenser discharges) it has been shown that an amount of work 



equal to 4 erg may be enough to cause excitation of a frog's nerve. 



* It would be more correct to say that the sensory mechanisms are not 

 affected by these chemical stimuli than that the sensory nerves are not 

 affected. For there is no proof that the absence of response is due to 

 inexcitability of the nerve-fibres and not to inexcitabihty of the centres 

 connected with them. 



38 



