CONDUCTIVITY AND EXCITABILITY OF NERVE 85 



being much more sensitive to changes in their normal metabolism, 

 as well as to any kind of injury, than the latter. This is, how- 

 ever, most conspicuous on comparing the excitability of different 

 parts of a motor organ (consisting of nerve-cells and fibres, along 

 with their peripheral terminations in the muscle), solely with 

 reference to the excitatory effects which each exhibits. It must 

 obviously depend upon the momentary state of excitability, or 

 conductivity, of the cellular elements interposed along the fibres, 

 whether an excitation on the distal side of the same will produce 

 any excitatory movement or no. And in fact we see that the 

 reflex functions of the spinal cord may suffer, or be entirely 

 abolished, under certain conditions, while excitability and con- 

 ductivity in the motor and sensory portions of the reflex arc are 

 not perceptibly affected. Luchsinger applied this unequal capacity 

 of resistance in central nerve-cells and fibres to prove the direct 

 excitability of the cord after local destruction of the reflex 

 functions. Various cold-blooded animals with a long spinal 

 column (snakes, blind-worms, tritons, etc.) were decapitated, the 

 fore-part of the body being then plunged into salt water heated 

 to 40 45, while the rest of the body was kept at normal 

 temperature. The warmth soon destroyed the reflex properties 

 of the cervical and dorsal medulla, while excitability and con- 

 ductivity were still presumably intact in the long medullated 

 fibres. If (as usually occurs) electrical excitation of that part of 

 the medulla which no longer discharges reflexes can cause move- 

 ments of the tail, these can only (according to Luchsinger) be 

 due to direct excitation of the longitudinal motor fibres of the 

 spinal cord. Schiff objected that the testing of reflex capacity 

 in this part of the body by stimuli applied to the skin is no sure 

 guarantee of the total destruction of the reflex functions of the 

 spinal medulla. He pointed out the possibility of "intra-medullary " 

 reflexes, which only fail to appear within the warmed segment, 

 because its muscles are rigored by the previous exposure to heat. 

 Schiff adduces experiments on bombinators and toads, in which, 

 after warming the entire cord exclusive of the peripheral end of 

 the cauda equina till the trunk-muscles were completely rigored, 

 the reflex excitability of the hind limbs still persisted. 



Nevertheless the low resistance in the gray matter of the 

 cord, as compared with that of the white mass of the fibres, is 

 indisputable. By this, inter alia, we may explain the fact that 



