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 develop- 

 ment 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 acquaint- 

 ance with those appliances by means of which elec- 

 tricity 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 elec- 

 tricity. 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 sul- 

 phuric acid (Fig. 334). 



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 libera- 

 tion of hydrogen, as expressed in the following formula: 



FIG. 334. AN 

 ELECTRIC CELL. 



Zn 



H 2 SO 4 = ZnSO 4 + H 2 . 



The zinc sulphate passes into the solution, while the hydrogen accumulates 

 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 



671 



