58 



EXPERIMENTAL PHYSIOLOGY 



Secondary contraction from the heart. Lay the nerve of a muscle-nerve 

 preparation upon the beating heart of the frog. If the preparation is very 

 excitable the muscle will twitch with each beat of the ventricle. If the heart- 

 beat and the twitch are simultaneously recorded on a drum 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. 



Measurement of demarcation current ; Capillary electrometer. The capillary 

 electrometer consists of a thread of mercury, which is forced by pressure from 

 behind for a certain distance along a glass tube drawn out to a capillary ter- 

 mination ; the free end of the capillary is filled with dilute sulphuric acid and 

 dips into a vessel containing the same fluid. The capillary is observed with a 

 microscope. If the mercury and the sulphuric acid be now connected with wires 

 which are charged with electricity, there is produced a movement of the mercury 

 in the direction which the current would take i.e. from positive to negative 

 the ultimate extent of movement of the meniscus being, for the same electrometer, 

 proportioned to the difference of potential. From the direction and extent of 

 the movement the direction and electromotive force of any constant current 

 can therefore be gauged. The movements of the meniscus can also be photo- 

 graphed, and a graphic record thus obtained. 



PIG. 51. EXPERIMENT TO SHOW SECONDARY CONTRACTION, k, MERCURY KEY IN PRIMARY CIRCUIT ; 

 k 1 , SHORT-CIRCUIT KEY IN SECONDARY CIRCUIT ; a, FIRST MUSCLE ; 6, SECOND MUSCLE WITH 

 ITS NERVE LAID OVER THE FIRST. 



Join a pair of non-polarisable electrodes up in circuit with a capillary electro- 

 meter and Daniell cell through a rheochord and commutator in the manner shown 

 in the diagram (fig. 52), but with a piece of blotting-paper moistened with salt 

 solution placed across the electrodes instead of the muscle shown in the figure. 

 Put a short-circuiting key between the electrometer and the electrodes. Have the 

 short-circuiting key (k') shut at first so that the electrometer is short-circuited, 

 and the battery key (k) open. Bring the mercury meniscus into the field of the 

 microscope. Now open the short-circuiting key. If the electrodes are themselves 

 without current there will be no effect on the electrometer ; but usually there is 

 a slight effect, the direction and the amount of which should be noticed. Next 

 close the battery circuit, leaving the short-circuiting key open. Part of the 

 battery current is now sent through the electrodes and electrometer in a particular 

 direction (which can be reversed by the commutator), and there is a correspond- 

 ing movement of the mercury. Note the direction of this movement, and 

 by following out the wires from the battery determine with which part of 

 the electrometer the anode and kathode are respectively connected. By 

 means of the rheochord and commutator a definite proportion of the battery 

 current can be sent in either direction through the electrodes and through 

 any preparation with which they may be connected. Open the battery key 

 and close the short-circuiting key ; the meniscus should return to its original 

 position. 



Lay the muscle of a nerve- muscle preparation, which may have the distal end 

 cut or injured, upon the electrodes in place of the wet blotting-paper. Place it 



