86 ELECTRO-PHYSIOLOGY CHAP. 



the motor effect of direct excitation of the spinal cord appears the 

 more plainly in the muscles of the posterior extremities, in 

 proportion as the reflex excitability of the preparation is greater, 

 while it fails altogether when the latter is extinguished. Accord- 

 ing to Birge's experiments (supra), the same elements of the gray 

 matter of the lumbar cord (nerve-cells of the anterior horns) must 

 convey the excitation in the one case from centripetal, in the 

 other from centrifugal fibres, to the same fibres of the anterior 

 root. The reflex centre of the posterior extremities would there- 

 fore not merely be thrown into excitation from periphery to 

 afferent nerve-path, but also, as it were, possess two poles, one cen- 

 tral (the motor paths in the cord), the other peripheral (the sensory 

 fibres). All injuries that involve central conductivity are equally 

 prejudicial to the effects of reflex and of direct excitation of the 

 spinal cord. 



The extraordinary sensitiveness of the central nerve -cells 

 of warm-blooded animals to all disturbance of normal nutrition 

 indicates a priori (as is confirmed by Stenson's experiment) that 

 conductivity in the spinal cord will diminish and be abolished 

 much more rapidly in all paths interrupted by ganglion -cells, 

 under certain influences (especially anaemia or asphyxia), than 

 it is in cold-blooded animals, so that experiments on the 

 direct excitation of the cord involve much greater difficulties, 

 and fail much more easily, than in the latter. Moreover, it 

 is clear that the rapid and total interruption of the blood - 

 supply to the spinal cord (taking precaution to avoid the 

 cerebral circulation, and with artificial respiration), affords a 

 working method of ascertaining the uninterrupted conducting 

 paths in the medulla after exclusion of the others. For con- 

 ducting paths which are blocked in a few minutes by ausemia 

 cannot be regarded as the direct continuation of peripheral 

 nerve-fibres. It is far less probable that a medullated fibre of 

 the white columns of the cord should react differently to 

 anaemia from the like fibre of a peripheral nerve, than that the 

 functional disturbances that are induced by anaemia so much 

 earlier in the cord than in the peripheral nerves should be located 

 in the interpolated cells of the gray matter. And S. Mayer's 

 experiments on the effect of anaemia produced on the rabbit's 

 spinal cord by ligaturing the aorta high up, do in fact show that 

 vaso - motor fibres originate in the medulla oblongata, and 



