MUSCLE STIMULI. 



453 



also act as stimuli when they are applied to the" trans verse section 

 of a divided muscle. Among these may be named (1) the 

 mineral acids (Hcl, .1 per cent.) and many organic acids ; (2) 

 salts of iron, zinc, silver, copper, and lead ; (3) the neutral salts 

 of the alkalies of a certain strength ; (4) weak glycerine and 

 weak lactic acid, which only excite nerves when concentrated ; 



FIG. 183. 



I 



Du Bois-Reymond's Inductorium with Magnetic Interrupter. e. Primary coil through 

 which the primary, inducing, current passes, on its way through the electro-magnet (ft). 

 i. Secondaiy coil, which can be moved nearer to or further from the primary coil (c , 

 thereby allowing a stronger or weaker current to he induced in it. This induced current 

 is the stimulating one. 6. Electro-magnet, which, on receiving the current, breaks the 

 contact in the circuit of the primary coil by pulling down the iron hammer (A), and sep- 

 arating the spring from the screw of e. When it brings the spring in contact with the 

 point of the pillar (a), it also demagnetizes itself by "short-circuiting" the battery. 

 When tetanus is to be produced, the wires from the battery are to be connected with g 

 and d. When a single contraction is required, the magnetic interrupter is cut out by 

 shifting the wire from a to the binding screw to the right of/. * 



(5) bile also is said to stimulate muscle in much weaker solutions 

 than it will nerve fibres. 



4. Electric Stimulation. Electricity is the most convenient 

 form of stimulation, because we can accurately regulate the force 

 of the stimulus. The occurrence of any variation in the intensity 

 of an electric current passing through a muscle causes it to con- 

 tract. The sudden increase or decrease in the strength of a 



