CENTRAL NERVOUS SYSTEM. 221 



modify the spread of the impulses within the central system, when this sys- 

 tem is represented by the spinal cord of a reflex frog. Admittedly, there is 

 in the case chosen but a fraction of the central system. It has been shown that 

 all incoming impulses tend to spread over a large part of the central system. 

 In a reflex frog, therefore, the cord is cut off from the remote effect of im- 

 pulses which normally enter the system by way of cells located in the portion 

 removed. Moreover, in the complete nervous system the incoming impulses 

 tend to be transmitted to the cephalic end, and in some measure give rise to 

 impulses returning within the central system and affecting the efferent cells. 

 In a fragment of the central system like the cord such impulses taken up by 

 the central cells must pass so far as the axones are intact; but as these for the 

 most part end at the level of the section, such impulses are lost, in the physi- 

 ological sense, at that point. 



The fact, therefore, that the experiments with spinal reflexes are con- 

 ducted on a portion of the central system has two important physiological 

 consequences. In the first place, there are wanting incoming impulses, direct 

 or indirect, from the portion removed ; on the other hand, through the sec- 

 tion of the afferent axones, in their course within the central system, there is 

 a direct diminution in the number of the pathways by which the impulses 

 arriving at the cord may be there distributed. It is most probable that in 

 the frog, at least, the reduction of the central mass does not so much dimin- 

 ish the number of pathways by which the impulses may be immediately 

 distributed by way of the afferent and central elements, as it diminishes the 

 number of impulses which by way of the portion removed arrive at the 

 efferent cells and modify their responsiveness. 



The modification of the responsive cells under more than one impulse is 

 well illustrated by an experiment of Exner: 1 A rabbit was so prepared that 

 an electric stimulus could be applied to the cerebral cortex at a point the 

 excitation of which caused contraction of certain muscles of the foot. ( )ne 

 of these muscles was attached to a lever so that its contraction could be 

 recorded, and a second electrode applied to the skin of the loot overlying the 

 muscle. The discharging efferent cells in the cord were in this case subject 

 to impulses from two directions, one from the cortex and one from the skin 

 of the foot. With a current of given strength stimulation of the cortex alone 

 caused a contraction of the muscle, and stimulation of the skin of the loot 

 alone, a similar contraction. When both were stimulated simultaneously the 

 extent of the contraction was greater than when either was stimulated alone 

 If now the strength of the stimulus applied to the skin of the paw was so 

 reduced that alone it was inefficient, then a stimulus from the cortex which 

 produced a reaction as indicated by the firsl cortical stimulus in fig. 9 1 

 {A, a), put the efferenl cells in such a condition that the stimulus from the 

 skin (A, h, Fig. 1)4), applied within 0.6 of a second, produced a second con- 

 traction of the muscle, although alone the stimulus from the skin had proved 

 inefficient. Here the first efficient stimulus from the cortex had rendered 



' Exner: Archivfur die gesammte Physiologie, Bd. xxvii. 



