CHAP, ii.] 



THE CONTRACTILE TISSUES. 



51 



mechanical stimuli, while a single blow may cause a single spasm, 

 a pronounced tetanus may be obtained by rapidly striking suc- 

 cessively fresh portions of a nerve. With chemical stimulation, as 

 when a nerve is dipped in acid, it is impossible to secure a 

 momentary application; hence tetanus, generally irregular in 

 character, is the normal result of this mode of stimulation. In the 

 living body, the contractions of the striated muscles, brought about 

 either by the will or by reflex action, are generally tetanic in 

 character. Even very short sharp movements, such as a sudden 

 jerk of the limbs, are in reality examples of tetanus of short 

 duration. 



FIG. 9. THE MAGNETIC INTERBUPTOE. 



The figure is introduced to illustrate the action of this instrument as commonly 

 used by physiologists. 



The two wires x and y from the battery are connected with the two brass pillars 

 a and d by means of screws. Directly contact is thus made the current, indicated 

 in the figure by the thick interrupted line, passes in the direction of the arrows, up 

 the pillar a, along the steel spring b, as far as the screw c, the point of which, 

 armed with platinum, is in contact with a small platinum plate on 6. The current 

 passes from b through c and a connecting wire into the primary coil p. Upon its 

 entering into the primary coil, an induced (making) current is for the instant de- 

 veloped in the secondary coil (not- shewn in the figure). From the primary coilp 

 the current passes, by a connecting wire, through the double spiral, m, and, did 

 nothing happen, would continue to pass from m by a connecting wire to the pillar d, 

 and so by the wire y to the battery. The whole of this course is indicated by the 

 thick interrupted line with its arrows. 



As the current however passes through the spirals m, the iron cores of these are 

 made magnetic. They in consequence draw down the iron bar e, fixed at the end of 

 the spring 6, the flexibility of the spring allowing this. But when e is drawn down, 

 the platinum plate on the upper surface of 6 is also drawn away from the screw c, 

 and a similar platinum plate on the under surface of b is brought into contact with 

 the platinum-armed point of the screw /, the screws being so arranged that this 

 takes place. In consequence of this change the current can no longer pass from 6 to 



