SECTION I. 



CHAPTER I. 



PRELIMINARY TECHNIQUE. 



Electrical Stimulation. In experimental physiology it is 

 necessary in many instances to employ an artificial stimulus in 

 order to cause activity in the tissue to be studied. Many kinds of 

 stimuli are capable of doing this, but to most of them there are 

 objections when used on nerve or muscle. Mechanical, chemical 

 and thermal stimuli are all more or less injurious. Moderate 

 electrical stimulation on the other hand brings these tissues into 

 action without appreciable injury. As this is the means by which 

 the tissues are usually stimulated, we will discuss briefly the prin- 

 ciples involved in the more important pieces of electrical apparatus 

 employed. 



For a source of current either a dry cell or a dynamo may 

 be used. A dry cell is essentially a large zinc cup serving as 

 container in which is suspended a carbon cylinder. A small amount 

 of water containing ammonium chloride held by gelatin or some 

 inert substance fills the rest of the container. A dry cell is 

 essentially a wet cell, but with the water protected from spilling 

 by the gelatin "sponge' 4 '. Whenever the carbon and zinc are con- 

 nected by a conductor, electrical energy is set free by the solution 

 and is transmitted along the conductor. It has been found that 

 if a live muscle is placed in the electrical circuit that the tissue will 

 be stimulated at the time the circuit is made and again when it is 

 broken provided the current is great enough. 



Two important factors must be kept in mind in the use of an 

 electric current, the resistance of the tissue and the "pressure" 

 of the current. The unit of "pressure" is the volt. Therefore the 

 amount of pressure is called the voltage. By increasing the voltage 

 the penetrating power is increased and thus the physiological effect 

 augmented. If the resistance of the tissue is high as in dry skin, a 



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