MUSCLE-NERVE PHYSIOLOGY. 507 



Temperature. The irritability of muscle is increased by raising its 

 temperature slightly above that of the animal from which it has been 

 taken, while it is decreased by cooling. If, however, the temperature 

 be raised too high (45 C. for frog, 50 C. for mammal), the muscle en- 

 ters into a condition of heat rigor and its irritability is forever lost. 

 After cooling, unless the cold be too severe and prolonged, the irritabil- 

 ity returns as the temperature is raised. The effect of cold on irritabil- 

 ity is shown in the superficial muscles of the face in winter. 



Chemicals and Drugs. Most chemical substances cause a marked 

 alteration of irritability in muscle. In general terms, it may be said 

 that those which produce any effect at all at first increase and then 

 diminish irritability. 



Mechanical stimuli at first increase and then diminish the irritability 

 of muscle. If they are powerful enough, the muscle is destroyed. 



The Phenomena of Muscular Contraction. 



The power which muscles possess of contraction may then be called 

 forth by stimuli of various kinds, and these stimuli may also be applied 

 directly to the muscle or indirectly to the nerve supplying it. There are 

 distinct advantages, however, in applying the stimulus to the nerve, as 

 it is more convenient, as well as more potent. The stimuli are of four 

 kinds, viz. : 



(1.) Mechanical stimuli, as by a blow, pinch, prick of the muscle or 

 its nerve, will produce a contraction, repeated on the repetition of the 

 stimulus; but if applied to the same point for a limited number of times 

 only, as such stimuli will soon destroy the irritability of the preparation. 



(2.) Thermal Stimuli. If a needle be heated and applied to a muscle 

 or its nerve, the muscle will contract. A temperature of over 45 C. 

 (113 F.) will cause the muscles of a frog to pass into a condition known 

 as heat rigor. 



(3.) Chemical Stimuli. A great variety of chemical substances will 

 excite the contraction of muscles, some substances being more potent in 

 irritating the muscle itself, and other substances having more effect upon 

 the nerve. Of the former may be mentioned, dilute acids, salts of cer- 

 tain metals, e.g., zinc, copper and iron; to the latter belong strong 

 glycerin, strong acids, ammonia and bile salts in strong solution. 



(4.) Electrical Stimuli. For the purpose of experiment electrical 

 stimuli are most frequently used, as the strength of the stimulus may be 

 more conveniently regulated. Any form of electrical current may be 

 employed for this purpose, but galvanism or the induced current is usu- 

 ally chosen. 



