GENERAL PHYSIOLOGY OP NERVE-CENTRES. 781 



current is closed the stimulation occurs only at the negative pole at the 

 moment when electrotonus takes place. On the other hand, with the 

 breaking shock, or when the current is opened, the point of greatest 

 stimulation is at the anode and coincides with the disappearance of the 

 electrotonus. 



The contraction which is produced on opening and closing a constant 

 current varies not only with the direction but with the strength of the 

 current. Very feeble currents produce a contraction only on the closing 

 of the current, both with an ascending and descending current, from the 

 fact that the occurrence of kathelectrotonus produces a greater effect on 

 the irritability of the nerve than the disappearance of anelectrotonus. 

 When the current is increased in strength contractions are produced 

 both on opening and on closing the current with either an ascending or 

 descending current. If, again, the current is greatly increased, contrac- 

 tion is only produced on closing the descending current and on opening 

 an ascending current. This is to be explained by the fact that with very 

 strong currents the entire intra-polar portion of the electrotonic nerve is 

 incapable of conducting an impulse, and, as a consequence, ascending- 

 currents can cause only an opening contraction. These results may be 

 expressed in the following table, in which R = rest, C = contraction : — 



Ascending. 



Descending. 



On On 



On On 



Closing. Opening. 



Closing. Opening. 



. C R 



C R 



. C C 



C C 



. R C 



C R 



Weak, 



Medium 



Strong, 



IV. GENERAL PHYSIOLOGY OF THE NERVE-CENTRES. 



The nervous system, as already indicated, consists of a combination 

 of ganglion cells united together by nerve-fibres. The second element 

 of the nervous tissue, or nerve-cell, is a mass of protoplasm supplied 

 with a nujleus and nucleolus, from which originate at least two proto- 

 plasmic strands or nerve-fibres, which serve to bind the different elements 

 of the nervous system together. Unfortunately, it is not possible to ob- 

 tain as decisive results by experimentation as to the functions of the 

 nerve-centres as may be determined as regards the nerve-fibres. 



The properties of the central organs of the nervous system can, 

 therefore, be only indirectly determined. The nerve-centres, by which is 

 meant simply a collection of nervous ganglia, may be divided into two 

 general classes ; one group is located on the surface of the body, and it 

 is adapted to the reception of stimuli originating in various external 

 influences brought to bear on the body surface ; the other group of 

 nerve-centres is located in the central nervous system, so-called, — in other 

 words, the spinal cord and brain, or the cerebro-spinal^axis. In addition 



