ELECTRICAL CHANGES IN TISSUES 



WE have already had occasion to refer to the electrical changes occurring in 

 nerves, muscles, and glands when excited to activity. With the exception of the 

 electric fish, these responses are chiefly of interest on account of the light they 

 throw on the physiological processes themselves, and they frequently serve as a 

 valuable means of investigating these processes. Perhaps the most striking of 

 these cases is the use now made of the string galvanometer in researches on the 

 heart, both in health and in disease. 



At the time when physiological phenomena began to be systematically worked 

 at from the point of view of accurate measurements, the electrical phenomena 

 naturally attracted much attention, on account of the methods available for the 

 precise determination of the value of electrical currents, and much valuable work 

 was done at that time. 



METHODS OF INVESTIGATION 



Since so much depends on the instruments used, it is worth while to give some 

 attention to the principles involved in the construction and use of these instruments. 

 Many of these principles will be found to apply to the methods used in other 

 investigations, such as the measurement of sudden changes of pressure, as in the 

 heart beat. 



The two factors of electrical energy, capacity and intensity, lead to the division 

 of the instruments used into two main classes ; those for the measurement of 

 current, galvanometers, and those for the measurement of potential, electrometers. 

 But, as we shall see, the galvanometers used in work on the electrical changes of 

 tissues allow, as a rule, so small a current to pass that they behave practically as 

 electrometers, and the indications given by the two kinds of instrument are very 

 much the same. The reason why the galvanometers have so high a resistance is, 

 of course, on account of the very high resistance of the external circuit, the tissues, 

 and the galvanometer gives the largest deflection when its resistance is equal to 

 that of the outer circuit. 



Galvanometers. All of these depend on the relative movement of a magnet 

 and a wire through which a current flows. In one type, such as that known as 

 the Kelvin form, the magnet moves under the influence of a current in a wire 

 surrounding it ; in the other type the magnet is stationary, and may be either a 

 permanent magnet, as in the D'Arsonval pattern and many forms of commercial 

 ammeters and voltmeters ; or it may be an electro- magnet, as in the string galvano- 

 meter of Einthoven. In this second type, the wire conveying the current moves. 

 In the D'Arsonval instruments the wire is in the form of a light rectangular coil ; 

 in the Einthoven form it consists merely of a very fine wire stretched between 

 the poles of a powerful electro-magnet. When a current passes through the wire 

 of the string galvanometer, the wire is deflected to one side or the other, according 

 to the direction of the current, and the movement is magnified by a microscope 

 and photographed by projection on to a slit, behind which is a moving sensitive 

 surface. 



Inertia of Moving Parts. When an electrical change lasts a very short time, 

 it is plain that it would be able to move a very light system when it was unable 



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