APPENDIX. 



PHYSIOLOGIC APPARATUS. 



The study of the physical and physiologic properties of muscles and 

 nerves necessitates the employment of some stimulus which, when applied to 

 either tissue, will call forth a contraction of the muscle, or the development of 

 a nerve impulse in the nerve. The most convenient stimulus is electricity, 

 for the reason that, with appropriate apparatus, its intensity and duration 

 can be graduated with the utmost nicety. Moreover, it does not destroy 

 the tissues, as do many chemic, physical, and mechanic stimuli. 



It is therefore necessary that the student should have a practical ac- 

 quaintance with those appliances by means of which electricity is generated, 

 applied and controlled. 



The electric cell is an apparatus composed of different elements, which, 

 by virtue of chemic actions taking place among them, generate and conduct 

 electricity. In its simplest form an electric cell consists of two metals 

 zinc and copper, or carbon, or platinum, etc., immersed in an exciting fluid, 

 usually dilute sulphuric acid (Fig. 354). 



The zinc element is the one acted on chemically by the sulphuric acid, 

 and at the expense of which the electricity is maintained. It is known as the 

 generating element. The copper is the collecting and conducting element. 



With the immersion of these elements in a solution of H 2 SO 4 a chemic 

 action at once takes place between the zinc and the acid, with the formation 

 of zinc sulphate and the liberation of hydrogen, as expressed in the following 

 formula: 



Zn + H 2 S0 4 = ZnS0 4 + H 2 . 



The zinc sulphate passes into the solution, while the hydrogen accu- 

 mulates on the surface of the copper element. 



As all chemic action is accompanied by the development of electricity, 

 it can be shown by appropriate means that this is the case at the surface of 

 the zinc. Such a combination is the means of establishing a difference of 

 potential between two points; the point of highest potential being the surface of 

 the zinc or the positive element, the point of lowest potential being the copper 

 or the negative element. So long as the elements remain unconnected 

 there is no movement of electricity, no current. 



If the ends of the elements projecting beyond the fluid are connected by 

 a copper wire, a pathway or circuit is established, and a movement of the 

 electricity takes place. As electricity flows from the point of high to the 

 point of low potential, it follows that inside the cell the current flows from 

 the zinc to the copper, and outside the cell from the copper to the zinc. 

 Such a current is termed a continuous, a galvanic or a voltaic current. Inas- 

 much as there is a progressive fall in potential between the Merriest and 



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