GENERAL PHYSIOLOGY OF MUSCLE AND NERVE TISSUE 69 



much too strong to be permitted to traverse the instrument. In 

 testing the instrument with a Daniell or any similar cell use a rheo- 

 cord joined as shown in Fig. 41, and make the first test with the 

 slider almost touching the zero post, and subsequent tests with 

 small increments until the movements of the meniscus are con- 

 siderable in extent, yet not so much as to carry it out of the field 

 of the microscope. 



If it is desired to make quantitative tests of electromotive force 

 the electrometer may be graduated. To accomplish this it is neces- 

 sary to have a micrometer in the ocular of the microscope, so that 

 the position of the meniscus may be accurately determined. It is 

 also necessary to have some means of measuring the force of dis- 

 placement of the meniscus. This may be done by means of a mercury 

 manometer, shown in Fig. 40 as a part of the electrometer. Pressure 

 exerted on the bulb is measured by the manometer. The amount 

 of pressure required to bring the meniscus back to its original posi- 

 tion after the opening of the key K is proportional to the electro- 

 motive force that displaced the meniscus during the opening of 

 the key. 



By testing a series of known values and taking the manometer 

 readings one may easily determine the relation between volts and 

 millimetres of mercury pressure. 



XIX. ELECTROMOTIVE PHENOMENA OF ACTIVE MUSCLE. 



(A) The physiological rheoscope, or the rheoscopic frog. 



(B) Electromotive force detected by the electrometer. 



In the process of dissecting out a muscle-nerve preparation (the 

 classical form) one is likely to drop the cut-off central end of the 

 sciatic nerve upon the gastrocnemius muscle. Should this occur a 

 contraction of the muscles is almost sure to occur. Galvani made 

 this observation and cited it as a proof that electricity exists in 

 animal tissues. 



A classical experiment well adapted to demonstrate the difference 

 of electric potential in living tissues is that known as the rheoscopic 

 frog. 



(A) The Rheoscopic Frog. 



1. Appliances. Frog; two glass slides, 1 inch by 3 inches; oper- 

 ating case. 



2. Preparation. Pith the frog. Make two classical muscle-nerve 

 preparations. Place the two glass slides end to end upon the table 

 (as shown in Fig. 42), with a muscle on each disposed as shown in 



