260 PHYSIOLOGY CHAP. 



may set up a vigorous and widespread reaction, with great ex- 

 penditure of energy. 



As we know nothing of the physiological process which is the 

 material basis of nerve excitation, we are a. fortiori ignorant of 

 the physiological process which underlies the excitation of the 

 centres. It can only be said that from the subjective, psycho- 

 logical point of view, it may be distinguished as conscious and 

 unconscious, according as it is accompanied or unaccompanied by 

 changes in the ego and the state of consciousness. From the 

 objective physiological point of view it may be either reflex or 

 automatic, i.e. evoked by impulses that reach the centre from the 

 periphery by afferent paths, or by such as arise within the centre 



itself, and are sent out peri- 

 pherally to the motor apparatus. 

 Both reflex and automatic acts 

 may, of course, be either con- 

 scious or unconscious. 



We have so far always 

 spoken of centres or of central 

 grey matter in contrast to the 

 peripheral nerve-fibres, but this 

 general expression includes two 

 quite distinct structures, the 

 ganglion elements or nucleated 

 nerve -cells, and the extra- 

 cellular fibrillary network. 

 Here, again, the question crops 

 up : is the central process (re- 



Fio. 163. One of the unipolar n<>rve-cdls that Hex Or automatic, 



"liS: ) f U "' ' or unconscious) seated in the 



ganglion cells or in the extra- 



cellular network of fibrils ? From the morphological point of view 

 the matter is still as we have seen sub judice (pp. 180 et seq.}\ 

 but we must now review the physiological arguments that bear on 

 one or the other of these hypotheses. 



In support of the opinion that the ganglion cell is only a 

 trophic centre, a reservoir for the nerve currents, while the central 

 activity of the system develops outside the cell, in the elementary 

 neuro-fibrillary network of the grey matter, Bethe (1897-8) adduced 

 an experiment made upon Carcinus maenas, a crayfish. The 

 muscles of the antennae of this crustacean are innervated by 

 neurones which (as shown by the diagram, Fig. 163) recall the 

 unipolar cells of the spinal ganglia of mammals. At a considerable 

 distance from the pear-shaped cell body the nerve process divides 

 into two branches, one of which is in connection with the dendrites 

 of other neurones or neuropile, and forms the cellulipetal path, 

 the other runs to the muscles of the antennae and forms the 



