502 MANUAL OF PHYSIOLOGY. 



the making, when the stimulus starts from the cathode, because 

 the impulse has a less distance of nerve to traverse in the former 

 case. 



In ordinary experiments on nerve, a constant current, i.e., one 

 coming directly from a battery, is seldom used, because there is 

 no means of regulating or varying the strength of the stimulation, 

 and it is not convenient to make and break the current in order to 

 excite the tissue continuously. And further, the more rapid 

 current induced in one coil of wire the secondary coil by the 

 making or breaking of a current passing through another coil 

 the primary coil is more effective and suitable for physiological 

 purposes. The strength of the induced current being approxi- 

 mately in inverse proportion to the square of the distance between 

 the two coils moving the secondary away from the primary coil 

 gives a ready means of varying and regulating the strength of 

 the stimulus without any special care being devoted to the exact 

 strength of the element used. 



Du Bois Reymond's Inductorium is the instrument commonly 

 used in physiological laboratories. In it the secondary coil can 

 be moved away from the primary on a slide which is graduated, 

 and the primary current may be made to pass through a magnetic 

 interrupter so as to cause a rapid succession of breaks and makes, 

 and thus give a series of stimulations one after another, which is 

 necessary in order to produce tetanus. A drawing and further 

 description of the instrument will be found at p. 451. 



VELOCITY OF NERVE FORCE. 



It has already been stated that nerve fibres are capable of con- 

 ducting impulses in either direction from or to the nervous 

 centres the position and the character of the terminal organs 

 determining the direction in which the nerve force usually travels. 

 In the ordinary peripheral nerves there are generally both kinds 

 efferent and afferent fibres carrying impulses in different direc- 

 tions without interfering with one another. 



When we reflect that the passage of an impulse along a nerve 

 is brought about by a molecular change in the axis cylinder, we 

 are at once struck with the rapidity with which impressions are 



