GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 67 



temperature the irritability becomes abnormally high when they are warmed 

 to ordinary room-temperature. 



Effect of Chemicals and Drugs. The activity of nerve and muscle proto- 

 plasm is markedly influenced by even slight changes in its constitution. If 

 a nerve or muscle be allowed to lie in a liquid of a different constitution from 

 its own fluid, and especially if such a liquid be injected into its blood-vessels, 

 an interchange of materials takes place which results in an alteration of the 

 constitution of the tissue, and a change in its irritability. Indeed, the only 

 solutions which fail to alter the irritability are those which closely resemble 

 serum and lymph. Fluids having other than the normal percentage of salts 

 have a marked effect, while the absence of proteids appears to have little 

 influence unless continued for a considerable time. These facts have been most 

 clearly demonstrated in experiments upon the nature of fluids essential to the 

 maintenance of the activity of isolated heart muscle. Most drugs and chemicals 

 capable of influencing the irritability of nerves first increase and later destroy 

 the irritability. It is said that sensory fibres are less susceptible to chemical 

 stimulation than motor, but this is not certain. If the change in the chemical 

 condition of the nerve or muscle be a rapid one, it is usually accompanied by 

 the phenomenon of excitation ; if more gradual, the irritability alone is altered. 

 The simple withdrawal of water from a motor nerve, by drying, or by strong 

 solutions of neutral alkaline salts, urea, glycerin, etc., causes first an increase 

 and later a decrease and loss of irritability. The increase of irritability is 

 frequently accompanied by active irritation, the muscle in connection with the 

 nerve showing rapid irregular contractions as different fibres of the nerve are 

 one after the other affected. If the drying has not been too long continued, 

 the irritability may be restored by supplying water. On the other hand, 

 imbibition of distilled water may, by altering the relative amount of salts, or 

 from mechanical causes, produce a lessening of irritability. If water be 

 applied to the tissues by being injected into the blood-vessels, it first excites 

 contractions and later causes a decline of irritability. Veratria, eserin, 

 digitalis, alcohol, chloroform, ether, sublimate, mineral acids (except phosphoric), 

 many organic acids, free alkalies, most salts of the heavy metals, destroy the 

 irritability of nerves and muscles, as a rule after first producing increased 

 excitability. Carbon dioxide, either because it is an acid or because of some 

 specific effect, acid potassium phosphate, and lactic acid, lessen the irritability. 

 Neutral potash salts, if concentrated, rapidly kill but excite less than do soda 

 compounds. Many gases and fumes chemically irritate and kill nerve and 

 muscle protoplasm. 



Ammonia, neutral salts, carbon bisulphide, and ethereal oils may destroy 

 the irritability of nerves without causing excitation, at least not sufficiently to 

 produce visible contractions of the muscle. If directly applied, however, 

 these substances excite muscles. 



A sodium-chloride solution, of a strength of 6 parts per 1000 of distilled 

 water, has been called the physiological solution because it was supposed to 

 have no effect on the irritability of nerves and muscles ; but late experiments 



