PHYSIOLOGICAL GRADIENTS. 169 



(Child and Hyman, '19; Child, '19^) in the plate rows of Cteno- 

 phores (Child, 'lye), the growing arms of echinoderm larvae, 

 branchiae and sensory tentacles of various annelids, the growing 

 tail of the ascidian and the amphibian tadpole, etc. Dr. Hyman 

 has found that the embryonic heart of the chick and of the fish 

 represents a susceptibility gradient with the high region at the 

 sinus end. But the most extensive work on the metabolic gra- 

 dient of any organ is that of Alvarez and his assistants on the 

 vertebrate alimentary tract. 1 They have found in the small intes- 

 tine a gradient in irritability, latent period, tone rhythm, conduc- 

 tion and susceptibility to various drugs and gradients in at least 

 some of these conditions in the wall of the stomach and colon. 

 Tashiro ('17 and earlier papers) has found a gradient in CO 2 

 production in certain nerves, the direction of functional conduc- 

 tion being down the gradient and in certain of these nerves a 

 susceptibility gradient has been observed (Child, '140). 



In an investigation of the respiration of ground nervous tissue 

 C. G. MacArthur and Jones ('17) have found differences in rate 

 of respiration in different parts of the central nervous system 

 which indicate the existence of a gradient in rate of respiration. 

 The rate of respiration is highest in the cerebrum and decreases 

 in the various parts in the following order : cerebellum, midbrain, 

 medulla, corpus callosum, spinal cord, nerve. The authors find 

 that gray matter consumes about twice as much oxygen and pro- 

 duces about one and one half times as much CO, 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 

 constitute highly significant evidence for the existence of an axial 

 gradient in rate of respiration in the central nervous system. 



All the evidence is in agreement as regards the existence of 



1 Alvarez, '14, '150, b ; '\6a, b ; '1701, b; ':8a, b, c ; Alvarez and Starkweather, 

 'i8a, b, c, '19; Alvarez and Taylor, 'ija, b; Taylor and Alvarez, '17. 



