504 MANUAL OF PHYSIOLOGY. 



the anode near it, the contraction after a breaking shock, when the 

 stimulus starts from the anode, will occur sooner than that which 

 follows 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 stimula- 

 tion, 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 physio- 

 logical purposes. The strength of the induced current being 

 approximately 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 the 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. 453. 



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 

 directions without interfering with one another. 



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



