THE MUSCULAR SYSTEM. 407 



uted to them. The amount of common sensibility in muscles is not 

 great; for they maybe cut or pricked without giving rise to severe pain, 

 at least in their healthy condition. But they have a peculiar sensibility, 

 or at least a peculiar modification of common sensibility, which is shown 

 in that their nerves can communicate to the mind an accurate knowl- 

 edge of their states and positions when in action. By this sensibility we 

 are not only made conscious of the morbid sensations of fatigue and 

 cramp in muscles, but acquire, through muscular action, a knowledge 

 of the distance of bodies and their relation to each other, and are en- 

 abled to estimate and compare their weight and resistance by the effort 

 of which we are conscious in measuring, moving, or raising them. 



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 through the 

 nerves, 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 nerves, 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 100 F. 

 (37.8 C.) 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 usually 

 chosen. 



(1.) Galvanic currents are usually obtained by the employment of a 

 continuous current battery such as^that of Daniell, by which an elec- 

 trical current which varies but little in intensity is obtained. The bat- 

 tery (Fig. 284) consists of a positive plate of well-amalgamated zinc im- 

 mersed in a porous cell, containing dilute sulphuric acid; and this cell 

 is again contained within a larger copper vessel (forming the negative 



