Chap. VII.] 



CONTRACTION. 



73 



contraction, create a diiference of potential between 

 the part of the nerve that touches the tendon and the 

 part touching the muscle fibre of mi, and this difference 

 of potential irritates the nerve of ra2, causing its 

 contraction. Further, if the nerve of m-2 be laid on 



.--r 



Fig. 40. Arrangements for showing Secondary Contraction of Muscle. 



mi without any precaution as to position, and 

 tetanised, mz will be thrown into tetanus also. 



Mechanical stimulation of nerve may be 



effected by pinching the nerve, pricking, or beating 

 it, a contraction of the muscle resulting. An electro- 

 magnetic arrangement for producing tetanus by a rapid 

 series of such mechanical irritations was devised by 

 Heidenhain, and is called the TETANOMETER. It is a 

 modification of the Wagner hammer described on page 

 43, and is shown in Fig. 41. It consists of a block 

 of ebonite, on which there stands erect an electro- 

 magnet, consisting, as usual, of two soft iron cores 

 wound round with insulated coils of moderately thick 

 copper wire, so wound that on the passage of a current 

 the two pillars become like a horse-shoe magnet, of 

 which one is north pole, the other south. The keeper 

 of this magnet is a piece of soft iron L, which has 

 attached to it the lever 7iL,s"i. The lever is supported 



