ADVANCED EXPERIMENTAL 

 PHYSIOLOGY 



CHAPTER I 

 THE PROPERTIES OF NERVE 



Although the nerve is not a unit, but a branch of a nerve-cell 

 which conducts an impulse to, or from, the periphery, it is con- 

 venient to examine its characteristics apart from its nerve-cell. 

 The chief of these are excitability and conductivity. Excitability, 

 or, as it is sometimes called, irritability, is the response to a 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, but its use is not free from sources of error. 



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 

 gastrocnemius 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 should be 

 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 

 conduced through the ligatured nerve. The moistened thread 



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