IRRITABILITY. 449 



if they dip into e and /, a will be connected with e, and through 

 the cross-wire with d, and 6 will be connected with /, and through 

 the cross-wire with c. This arrangement permits the battery- 

 current to pass by way of a and e down the nerve, and back to 

 the battery by way of d and b ; or from the battery through a, e, 

 and d, and up the nerve and back to the battery by way of c, /. 

 and b. 



In Fig. 253, B, the cross-wires have been removed, and, as 

 shown by the diagram, the current can be sent through c and d 

 to one part of the nerve, or through e and / to another, by rocking 

 the bridge back and forth. 



Myographs. In order properly to study the effects of the 



FIG. 255. Spring myograph : A, B, iron uprights, between which are stretched 

 the guide-wires on which the travelling plate a runs ; fc, pieces of cork on the guides 

 to check gradually the plate at the end of its excursion and prevent jarring ; 6, 

 spring, the release of which shoots the plate along ; h, trigger-key, which is opened 

 by the pin d on the frame of the plate (Stewart). 



induction shocks upon the nerves and muscles it is necessary to 

 have some method of recording the movements of the muscles which 

 these shocks produce. Such an instrument is the myograph. The 

 nerve-muscle preparation has already been described. The tendon 

 of the muscle is attached to a lever, and to this latter is attached a 

 writing-point, which rests against a piece of paper wrapped around 

 a revolving drum, the paper revolving with the drum. When the 

 muscle contracts, it raises the lever and an upward line is made by 

 the point on the paper ; when the muscle relaxes, the lever falls 

 and the point makes a descending line. Inasmuch as this drum 

 is revolving all the time, these lines take the form of curves. 

 Such a record is a myogram or muscle-curve, and may be preserved 



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