FORMS OF STIMULI FOR MUSCLE OR NERVE 



443 



however, in applying the stimulus to the nerve, as it is more convenient, as 

 well as more potent. The stimuli which will produce contraction in a muscle 

 are: 



1. Mechanical Stimuli. A blow, pinch, prick of the muscle or its nerve 

 will produce a contraction, repeated on the repetition of the stimulus. If 

 applied to the same point for a number of times such stimuli will soon destroy 

 the irritability of the preparation. 



2. Thermal Stimuli. If a needle or glass rod be heated and applied to a 

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

 will cause the muscles of a frog to pass into a condition known as heat rigor. 

 The sudden change of temperature acts as a stimulus. 



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 certain metals, e.g., 

 zinc, copper, and iron; to the latter belong strong glycerin, strong acids, 

 ammonia, bile salts in strong solution, etc. 



4. Electrical Stimuli. Any form of electrical current may be employed 

 to stimulate a muscle to contract, but either galvanism or the induced current 

 is usually chosen. For experimental purposes electrical stimuli are most 

 frequently used, as the strength of the stimulus may be conveniently regulated. 

 In order that a stimulus shall be effective, it must have a certain amount of 

 energy and the application to the muscle must have a certain abruptness. 

 For example, a comparatively weak galvanic current suffices to stimulate a 

 muscle to action when suddenly applied in full force. But if the electric 

 current be applied very gradually, a current many times stronger will fail to 

 arouse contraction of a muscle. 



Necessary Apparatus used to Produce and Record a Muscle Contraction. Galvanic 



currents are usually obtained by the employment of a continuous-current cell such as 



that of Daniell, by which an electrical current which varies but little in intensity is obtained. 



The cell (figure 314 A) consists of a positive plate of well-amalgamated zinc im- 



A B 



Fig. 314. Diagram of a Darnell's Cell A, Dry Cell B, 



