22 PRACTICAL PHYSIOLOGY 



if a moistened finger of the other hand touches the other pole of the 

 secondary coil. In this case the human body acts like a condenser 

 charged with electricity, which by its approach can stimulate muscle 

 or nerve. Further, if the nerve be ligatured between the electrode and 

 the muscle, or cut across and the two cut ends laid over each other, 

 which will prevent the passage of a nervous impulse along it, contrac- 

 tion of the muscle is still produced, because the discharge takes place 

 along the whole length of nerve and muscle between the electrode and 

 the point by which the muscle is connected to the earth, so that any 

 irritable tissue in the course taken by the charge is stimulated. 



If, however, the muscle and nerve preparation is laid on an ordinary 

 moistened muscle-board, the insulation is so slight that one electrode, 

 connecting the nerve and the secondary coil, will by itself cause the 

 muscle to contract. 



It is in order to guard against accidental stimulation of muscle and 

 nerve by unipolar action that a Du Bois key must always be placed in 

 the secondary circuit, and must always be kept closed except when the 

 tissue is being intentionally stimulated. The brass bridge of the key, 

 which has many thousands of times less resistance than the tissue 

 between the electrodes, affords a perfect closure of the secondary 

 circuit and prevents static electrification of the electrodes. 



Errors from unipolar action are liable to take place, especially in the 

 study of the electromotive phenomena of muscle and nerve by the 

 electrometer and galvanometer (see Chap. IX., Part III). 



CHAPTER III. 

 A SINGLE CONTRACTION OF A GASTROCNEMIUS MUSCLE. 



IN order to study the contraction given by a muscle in response to 

 a single stimulus, it is not sufficient to inspect the curved line traced 

 by the myograph-lever on a revolving drum. It is also necessary to 

 study the length of time occupied by the whole twitch and the time- 

 relations of different parts of it. For this purpose a time-tracing must 

 be simultaneously recorded by a special apparatus, which generally 

 takes one of two forms. 



(1) The Tuning Fork; to one prong of this a writing point, similar 

 to that on the myograph-lever, is attached. With the writing point 

 lightly touching the blackened surface of the drum, a sharp tap is 

 given to the fork, and the drum set in motion ; care must be taken that 

 the drum does not make more than one revolution, otherwise the time- 



