vni CONDUCTIVITY AND EXCITABILITY OF NERVE 79 



of the spinal cord, while it lias little appreciable action on nerve 

 and muscle ; anaesthesia notably attacks the central structures 

 first, and the heart, peripheral nerves, and muscles at a later 

 period only. The same facts are met with when the temperature 

 is raised above a certain limit. Finally (and this is perhaps the 

 most characteristic), the amount of gas contained in the blood is 

 of the utmost importance to the excitability of the nervous centres, 

 which, more especially in warm-blooded animals, are so extra- 

 ordinarily sensitive to changes in their normal metabolism 

 (whether from anaemia or from dyspnoeic condition of the blood) 

 that they can, in this respect, hardly be compared with the peri- 

 pheral nerves and muscles. 



Accepting the highly probable assumption that the central 

 and peripheral nerve-fibres are, in the main, alike in physiological 

 properties, as also in structure and origin, it is easy to understand 

 why the motor consequences of direct excitation of the central 

 organs, and more especially of the spinal cord, should differ 

 in many respects from those of direct excitation of the peri- 

 pheral motor nerves, and should appear to depend essentially 

 upon the same conditions as reflexly-provoked movements. This 

 is the immediate outcome of the fact that each motor nerve-fibre 

 (of the anterior root) is the process of a nerve-cell, and that the 

 spinal cord can only affect it indirectly through the cell. Dis- 

 regard of this fact can alone account for the acceptance of the 

 singular theory that the central nerve-fibres conduct, but are not 

 excitable. 



The contrast is the more striking, since, on the one hand, 

 the central nervous organs, brain and spinal cord, seem to react in 

 such an extraordinary degree to the weakest natural " organic " 

 stimuli, and to propagate the excitation, while, on the other, the 

 nerve-fibres which participate in the structure of the nervous 

 centres are scarcely to be distinguished anatomically from those 

 in the peripheral nerves. 



On reviewing the experiments in point, we find that they 

 all aim at establishing that movements consequent on excitation 

 of the central organ are not reflexes, and ascertaining safe 

 objective criteria of sensibility in the animal. Thus Van Deen 

 tried to exclude the objection just made (re interpretation of 

 motor effects of excitation from the spinal cord) by a special 

 method which has since been frequently repeated. He exposed 



