78 NERVE-CENTRES. 



negativity of the cut surfaces (negative variation). The 

 time at which this happens depends on the distance 

 between the seat of excitation and the section. The rate 

 of propagation of the negative variation as measured by 

 .the "rheotome" (Bernstein), agrees with that of the trans- 

 mission of the excitatory state in a motor nerve, as 

 measured by the myograph. 



Electrotonic variation of the nerve current. If during 

 the passage of the voltaic current through the central part 

 of a length of nerve, the extra-polar parts are investigated, 

 electrical differences show themselves which have the same 

 direction as those which are produced by the current in 

 the inter-polar tract. 



FUNCTIONS OF NERVE-CENTRES. 



By the term "centre" are designated certain parts of 

 the Brain or Spinal Cord, respecting each of which it is 

 known that it is concerned in the regulation or control of 

 some one of the chemical or mechanical processes of the 

 living organism. Our knowledge of the limits and topo- 

 graphical relations of these parts, and of the channels 

 by which they mutually influence each other, and the 

 .organs over which they preside, is derived almost exclu- 

 sively from experiments on living animals. 



Nerve centres are excitable organs which owe their 

 physiological endowments to the nerve cells and reticulum 

 of which they consist, and to the connection of these 

 structures with afferent and efferent nerves. The excita- 

 tory state originates in them for the most part by pro- 

 pagation from afferent nerves, or from other centres, and 

 is propagated by them to other centres or to efferent 

 nerves. 



This process is designated " reflex action ;" a term which 

 is applied to any case in which the function of a mus- 



