288 THE GROWTH OF THE BRAIN. 



in the lumbar region of the cord. Yet the various 

 stimuli which have been enumerated have their points 

 of entrance into the central system not only at different 

 localities, but at those far removed from the lumbar 

 region ; nevertheless in every case reactions due to 

 impulses leaving the cord in this region are affected. 

 The diffusion of the incoming impulse is beautifully 

 shown by the studies on the quantity of blood in a limb 

 during sleep (Bardeen and Nichols). 



These experiments were made as follows : The arm 

 was encased in a plethysmograph, a glass cylinder filled 

 with water, and so arranged that the quantity of water 

 within it varied as the limb swelled or shrank. These 

 variations in the volume of the arm were recorded as 

 given in Figs. 66, 67. During sleep the arm normally 

 swelled. The curves begin at the left, the arm being in 

 a swollen condition. When the sleeper was disturbed 

 by a sound, though he was not awakened, yet the 

 nervous system was aroused sufficiently to give a 

 vigorous response, and the volume of the arm diminished 

 by the withdrawal of blood. This is indicated by the 

 depression of the curve. The value of these experiments 

 for the present discussion lies in the fact that they are 

 typical, and there is warrant for assuming that, if other 

 forms of stimuli were taken and their effects measured 

 by still other reactions, there would be found the same 

 indication of diffusion. The consideration of these facts 

 is important, especially by reason of the apparent oppo- 

 sition in which they stand to the doctrine of localisation, 

 with which they cannot fail to be contrasted. By way 

 of harmonising them, it should be remembered that the 

 facts of localisation are best demonstrated by the reac- 

 tions of efferent cells, whereas the phenomena of diffusion 

 pertain to the afferent and central elements. They are 

 two opposite aspects detectable in all complete reactions, 



