652 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



cord which controls the knee-kick are so modified that the extent of the 

 kick is noticeably altered. 



There is little doubt that if there were a means of measuring other motor 

 reactions and testing their variability as determined by variations in the 

 incoming stimuli, results concordant with these just given could be obtained. 

 They illustrate a fundamental condition in the reactions of the central system 

 namely, that every stimulus which falls upon it alters its responsiveness, and 

 that it is continually in a state of tension due to the effect of many stimuli 

 which we often fail to recognize. If we follow strictly the anatomical inter- 

 pretation, it appears, as a consequence of these observations, that any nerve- 

 impulse arriving over the afferent pathways can and does affect to a varying 

 degree all the efferent cell-elements, that there must be a pathway for the 

 nerve-impulses from some of the terminals of each afferent fibre to the neigh- 

 borhood of each cell giving rise to efferent impulses. 



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

 impulses 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. As to the first point, we should expect the afferent 

 nerves with the widest central connections, such as the olfactory, optic, and 

 auditory nerves, to be the most efficient in this respect, and this is the case. 

 Concerning the second, it is observed, for example, 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 sub- 

 stance of the spinal cord. 1 The peculiar change wrought in the nervous sys- 

 tem 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 When the strychnin solution was applied locally to the brachial 

 enlargement of the spinal cord of a brainless frog, a subsequent 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 of the leg muscles the efferent cells 

 controlling the muscles of the leg must be discharged and in the cue 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 

 1 Lovett : Journal of Physiology, 1888, vol. ix. 2 The Medical News, June 1, 1895. 



