LESSON IX 

 MUSCLE AND NERVE (Concluded) 



ELECTROTONUS. CATHODIC AND ANODIC EXCITATION. LAW OF 

 UNIPOLAR STIMULATION OF HUMAN MUSCLE AND NERVE 



1. Electrotonus. When a nerve is traversed by a constant or gal- 

 vanic current it undergoes certain physicochemical changes which be- 

 tray themselves in a change in its irritability at the points of entrance 

 and exit of the current. When the key is closed the excitation produc- 

 ing the nerve impulse is had at the cathode ( ). The anode (+) is 

 depressed at this time. On opening the key the excitation is had at the 

 anode, while the cathode is depressed. The former condition is known 

 as anelectrotonus, and the latter as catelectrotonus. Obviously, during 

 the passage of the constant current, we obtain only the condition of 

 anelectrotonus, while the catelectrotonus de- 

 velops in the wake of the current, i. e., after 

 it has been broken. 



Anodic Depression. Fasten two holders 

 to the rod in the moist chamber and secure 

 in each a non-polarizable electrode. A con- 

 'venient form of the latter is the clay-boot, 

 containing a rod of zinc. To begin with, 

 these boots should be immersed in normal 

 saline solution for a period of about twenty- 

 four hours to render the clay permeable to FIG. 40. METHOD OF 

 the electric current. They are then fastened c^ovSo^oFNEKVE^porter^ 

 in the holders. A zinc rod having been 



placed in each, the remaining space is carefully filled by means of a 

 dropper with a solution of zinc sulphate (see Fig. 7). 



Connect in series the binding-posts upon the rods, a key, and two 

 dry cells. Place a small tuft of cotton moistened with normal saline 

 solution upon each boot. Make a nerve-muscle preparation, and place 

 the nerve across the tufts of cotton, adjusting the distance between the 

 electrodes to suit the length of the nerve. Make and break the current 

 by closing and opening the key. With this strength of current a con- 

 traction is obtained only on the make of the current. Which electrode 

 is connected with the carbon (+) of the battery? Is the current ascend- 

 ing or descending in its direction? 



Arrange the inductorium for stimulation with single induction shocks. 

 Apply the hand-electrodes to any part of the nerve and accurately de- 

 termine the strength of the make shock which is required to give a just 

 perceptible contraction. Now close the key in the circuit of the constant 

 current, and while this current is allowed to flow, stimulate the region 



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