54 



PRACTICAL PHYSIOLOGY 



galvanometer to indicate electrical variations is 

 known as a rheoscopic preparation. 



Secondary contraction from the heart. Lay 



FIG. 36. SECONDARY CONTRACTION. 



the nerve of a muscle-nerve preparation upon the 

 beating heart. If the preparation is very excit- 

 able the muscle will be seen to twitch with each 

 beat of the ventricle. If the heart beat and twitch 

 are both recorded the twitch will be 

 found to slightly precede the beat 

 i.e., the electrical change'precedes the 

 mechanical ; this is seen best with a 

 cooled heart. 



Contraction without metals ; 

 Galvani's experiment. By means 

 of a glass rod loop up the nerve of a 

 nerve-muscle preparation, and allow FIG. 37 .-coNTRAc- 

 its cut end to come in contact either M^liI lTf 

 with the surface of its own muscle 

 (Fig. 37) or with other muscles. If the nerve is 

 very excitable there will be a contraction of its 

 muscle each time that the end of the nerve is 

 brought in contact with the muscle. This excita- 

 tion is caused by the closing through the nerve of 

 the demarcation current of the muscle. 



