46 PRACTICAL PHYSIOLOGY 



stimulus; a nervous impulse, the real nature of which is unknown, 

 is started at the point stimulated, and is transmitted or conducted 

 along the nerve. 



Nerves can be stimulated by electrical, mechanical, chemical or 

 thermal agents ; of these the most important in experimental 

 physiology is the electrical, for it can be finely graduated, is of 

 extremely short duration, and can be applied repeatedly without 

 damage to the nerve. The first experiments will therefore be the 

 electrical stimulation of nerve. 



The Electrical Stimulation of Nerve. An induction-apparatus is 

 arranged for single induction-shocks, and a simple pair of electrodes 

 is connected with the secondary coil by means of a Du Bois key. A 

 preparation of the sciatic nerve in its entire length and of the gastroc- 

 nemius muscle of a pithed frog is made, and near the origin of the 

 nerve is applied the pair of electrodes. 



On the passage of an induction-current through the electrodes the 

 nerve is stimulated, and an impulse is sent down the nerve, reaches 

 the muscle, and causes it to contract. This is indirect stimulation of 

 the muscle, and is, if a weak current be used, not due to an escape 

 of the electric current along the nerve towards the muscle. This is 

 proved by the following experiment. A moistened thread is tightly 

 tied round the nerve at a point between the electrodes and the muscle. 

 The passage of a weak induction-current of the same strength as that 

 previously used will stimulate the upper portion of the nerve, but the 

 nervous impulse will not pass through the block produced by the 

 thread. A breach in the physiological continuity has been produced, 

 and the nervous impulse is not conducted through the ligatured nerve. 

 The moistened thread would not prevent the passage of a purely 

 electric current. 



The response of the nerve to a stimulus bears within certain limits 

 a relation to the strength of the stimulus. This can be shown by the 

 following experiment. 



Maximal and Minimal Stimuli. The muscle of the preparation 

 is attached to a myograph, the lever of which is arranged to write 

 upon a drum covered with smoked paper. The electrodes are 

 placed between the muscle and the ligatured portion of the 

 nerve which was used in the previous experiment. The induc- 

 tion shock is made so weak that no response is obtained, and 

 is then gradually increased until a contraction is observed with the 

 break-shock Contraction does not follow each break-shock; the 

 stimulus is sub -minimal. The contraction is recorded as a vertical 

 line upon the stationary drum, and before each stimulation the drum 



