PHYSIOLOGICAL BASIS OF PATTERN 49 



much oxygen and produces about one and one-half 

 times as much CO 2 as white matter, and some of the 

 differences in respiratory rate in different parts of the 

 nervous system are doubtless due to differences in pro- 

 portion of white and gray matter. For example, the 

 relatively low rate of the corpus callosum is undoubtedly 

 associated with the fact that it consists of white matter, 

 nerve fibers, rather than cells. But the differences in 

 respiratory rate between cerebrum and cerebellum and 

 between midbrain, medulla, and spinal cord are scarcely 

 to be accounted for in this way. These differences con- 

 stitute highly significant evidence for the existence of an 

 axial gradient in rate of respiration in the central 

 nervous system. More recently MacArthur and Doisy 

 (1919) have demonstrated the existence of a gradient 

 in chemical differentiation in the brain. 



All the evidence is in agreement as regards the 

 existence of these physiological axial gradients. Thus 

 far every living physiological axis examined has given 

 evidence of the existence, at least in the earlier develop- 

 mental stages of such a gradient, and in many cases the 

 experimental methods show the presence of a gradient 

 w r here structural, or other directly visible indication of 

 its presence are absent. In whatever terms we may 

 finally interpret these gradients, there can be no doubt 

 concerning their existence. They are physiological 

 facts, and their significance for localization, differen- 

 tiation, and functional relation is already demonstrated. 



