CENTRAL NERVOUS SYSTEM. 217 



system of hibernating animals and all animals exhibiting well-marked periodic 

 variations in their habits of life. 



Variations in Diffusibility. — The degree in which any set of incoming 

 impulses is diffused and modifies the responsiveness of the central system 

 depends, in the first instance, on the physiological connections of the fibres 

 by which they travel, and, in the second, on the particular condition in which 

 the central cells happen to be found. It is observed that by means of drugs 

 it is possible to alter the diffusibility of incoming stimuli to an enormous 

 extent. Strychnin and drugs with a similar physiological action have this as 

 one of their effects. 



Influence of Strychnin. — The experimental study of strychnin-poisoning 

 shows the following relations : A frog poisoned by the injection of this drug 

 is easily thrown into tetanus whether the brain is intact or has been removed 

 previous to the injection. The drug is found to have accumulated in the 

 substance of the spinal cord. 1 The peculiar change wrought in the nervous 

 system is such that a slight stimulus will cause an extended and prolonged 

 tetanic contraction of the skeletal muscles — i. e., the diffusion of impulses 

 within the cord is very wide and (efficient to an unusual degree. The direct 

 application of strychnin to the spinal cord has been carefully studied by 

 Houghton and Muirhead. 2 AVhen the strychnin solution was applied locally 

 to the brachial enlargement of the spinal cord of a brainless frog, a subse- 

 quent stimulation of the skin of the arms produced tetanic contractions of 

 the arms, and later, after the poison had acted for a time, of the entire trunk 

 and legs. On the other hand, stimulation of the legs in such a case produced 

 a slight reflex or none at all. Since, in order to cause contraction Df the leg- 

 muscles, the efferent cells controlling the muscles of the leg must, be discharged 

 — and in the one case when the stimulus was applied to the arm region these 

 cells discharged so as to cause a tetanic spasm, while in the other, when the 

 stimulus was applied to the legs, they discharged only slightly — the alteration 

 in the cord produced by the drug must affect some other group than these 

 efferent cells. Since, moreover, a tetanus of the legs could be caused by the 

 stimulation of the skin of the arm, the application of the drug being to the 

 brachial enlargement only, it appears that the central cells, or those conduct- 

 ing the impulses entering by the dorsal root-fibres in the brachial region to 

 the nuclei of the lumbar enlargement, are probably affected ; and. further, 

 that it is on the bodies of these cells that the drug must act. since they 

 alone were in the locality at which the drug was applied. The application 

 of the drug to the dorsal root-ganglia and to the nerve-roots between the 

 ganglia and the cord proved to be without effect, SO that the two parts which 

 can possibly be influenced are the terminations of the sensory afferent nerves 

 within the cord ami some portion (the dendrites ?) of the central cells with 

 which these terminations are associated. lint whether the change is in both 

 these structures or only in one cannot now be determined. 



1 Lovett : Journal <;/' Physiology, L888, vol. ix. 

 ' : Mrtliral News, June 1, 1S95. 



