IRRITABILITY OF NERVES. 631 



with a certain degree of rapidity. As a result of the action of most of these 

 irritants the irritability of the nerves is at first increased ; then follows diminution 

 to the point of abolition. Chemical irritants have, as a rule, less effect on sensory 

 nerve-fibers than on motor fibers; so that chemical and thermal stimuli have to 

 a certain degree opposite effects upon motor and sensory nerves. According to 

 Griitzner the failure of chemical stimuli to exert any effect on sensory nerves 

 observed in most cases may, however, be due for the most part to want of simul- 

 taneousness of irritation of the individual fibers; and this view is supported by 

 the circumstance that substances having a rapid and powerful action are capable 

 under certain conditions of stimulating also centripetal fibers. Potassium and its 

 salts exert a stronger action upon the sensory nerves (causing pain) than sodium; 

 ammonium causes the most intense irritation. The painful effect of acids is 

 proportionate to their degree of acidity. Of the monatomic alcohols the higher 

 have a more intense action than the lower. Among stimuli of the motor nerves 

 are: (a) rapid dehydration either by dry air (surrounding the nerve with filter- 

 paper or suspending it over sulphuric acid) or by dehydrating fluids, such as con- 

 centrated solutions of neutral alkaline salts (sodium chlorid is said to stimulate 

 only the motor nerves in mammals; sugar, urea, also concentrated glycerin and 

 solutions of some metallic salts) . Subsequent addition of water at times causes 

 the contractions and spasm to disappear and the nerve may remain irritable. The 

 dehydration at first increases the irritability, but later it is diminished. Imbi- 

 bition of water decreases the irritability of the nerves. (6) Free alkalies, the min- 

 eral acids (not phosphoric acid), many organic acids (acetic, oxalic, tartaric, lactic), 

 most salts of the heavy metals. While acids generally have irritant effects only 

 in strong concentration, caustic alkalies have such effects in solutions down to 

 0.8 per cent, or even as low as o.i per cent. Neutral potassium-salts in concen- 

 trated form cause rapid destruction, but are less powerfully irritant than sodium- 

 combinations. Neutral potassium-salts, when used in dilute solution, at first 

 increase the irritability of the nerves and then diminish it, as can be demonstrated 

 especially by stimulation with the opening shock of an induced current, (c) The 

 anesthetics (ether, chloroform) and carbon dioxid in small amounts increase the 

 irritability of isolated nerves; in larger quantities they diminish it. The haloid 

 salts, especially the bromids, likewise diminish the irritability. Of the alkaloids 

 and narcotics some (opium, cocain, curarin, chloral hydrate) diminish the irrita- 

 bility in part, while others (morphin, strychnin, muscarin, atropin) are indifferent 

 in action. Other substances, such as dilute alcohol, bile, salts of the biliary acids, 

 and sugar, generally excite contractions at first, after which the nerve rapidly 

 dies. Ammonia, lime-water, solutions of some metallic salts, carbon disulphid, 

 and the ethereal oils destroy the nerve without stimulating it (thus without 

 inducing contractions in the frog-preparation) . Carbolic acid (which excites 

 convulsions on direct application to the spinal cord) has a similar action. These 

 substances have a directly irritating effect upon the muscle. 



Tannic acid has no irritating effect either upon the nerves or upon the muscles. 

 In general, the irritating substances must be applied to the nerves in more con- 

 centrated solution than to the muscles in order that contractions may result. 



Of chemically allied substances those act most intensely upon motor nerves 

 that have a higher molecular weight; for example sodium iodid acts more in- 

 tensely than sodium chlorid. 



The Physiological Stimulus. The nature of the physiological nerve-stimulus 

 in the normal body is not known. It passes either in a centrifugal direction, 

 from the central nervous system, as motor, secretory, or inhibitory impulses, or 

 in a centripetal direction, from the specific terminal expansion of the organs of 

 special sense and of the sensory nerves. The last-named class of stimuli are 

 conveyed to the central nervous organs, where they are perceived as sensations, or 

 they give rise by transference within the center to effects transmitted in a centri- 

 fugal direction and known as reflex processes. The individual physiological 

 motor stimulus occupies a longer time than the transitory irritation of an induced 

 current. It is not a uniform process, causing different effects in accordance with 

 the varying intensity and the more or less frequent repetition, but it is rather 

 a process exhibiting marked variability in the time of its occurrence and attain- 

 ing a duration as great as one-eighth of a second. 



Homologous and heterologous irritants and the law of specific energy are 

 considered on p. 813. 



Electrical Stimuli. The electrical current exerts its strongest irritant effects 

 upon a nerve at the time of its entrance into the nerve, and at the time of 



