214 EXPERIMENTAL PHYSIOLOGY 



for each two minutes should be plotted on coordinate paper to 

 show this phenomenon. 



2. After the hand and arm have been used to perform work, 

 such as dumb bell exercise, until almost fatigued. 



ELECTROCARDIOGRAPHY. 



Demonstration 4a. Electrocardiography. The electro- 

 cardiograph is essentially a very sensitive galvanometer. A fibre 

 of finely spun glass coated with a film of silver to render it elec- 

 trically conductive is suspended between the poles of a powerful 

 electromagnet. A current passing through the fibre or string, as 

 it is termed, causes a deflection of the string which varies with the 

 direction of the current. When the base of the heart is relatively 

 negative to the apex the current is directed upwards, when the 

 apex becomes negative the current passes downwards through the 

 string. 



By means of an arc light and a series of lenses a highly magnified 

 shadow of the string is cast upon a moving photographic plate 

 which descends at the desired speed in a carrier contained within 

 the camera box. In this way the movements of the string are 

 recorded. 



The arc lamp having been lighted and the shadow of the string 

 focussed upon the scale situated on the front of the camera, the 

 observed person is seated in the chair with each hand and a foot 

 placed in the respective electrode basins. The key, A, should be 

 in the short circuit position and the key B at "off" before the 

 subject is connected to the electrodes. (See Fig. 64a). The magnet 

 should be now excited by turning on the house current and the 

 current of the standard cell thrown into the circuit by closing the 

 battery switch. The current of the standard cell serves two 

 purposes, to compensate for the skin current and to standardize 

 the excursions of the string. The electrode switch E is next adjusted 

 for the particular lead from which it is desired to obtain a record. 

 The switch B may now be turned to " patient" and the switch A 

 turned from the short-circuit position to 1/100. A fraction of the 

 current from the subject is now passing through the string and, 

 though the cardiac deflections, if seen at all, will be very minute, 

 the string moves from the zero point and takes up a new position. 



