THE CIRCULATION OF CEREBRO-SPINAL SUBSTANCE. 583 



us to plainly indicate that the cerebral circulation is governed, 

 as is that of any other organ, ly the posterior pituitary body, the 

 vasomotor nerves of the pial vessels ~being terminals of the general 

 motor system. 



This only furnishes us, however, the functional mechanism 

 of the passive stage. In other words, the nervous energy de- 

 veloped owing to the presence of a given proportion of oxidizing 

 substance (brought into contact with the myelin through the 

 tonic vascular contraction insured by the general motor sys- 

 tem) only causes the entire brain to create the nervous energy 

 which, as we have seen, is essential to its own life. But how 

 is the active stage incited in any one part of the cerebrum? 



As is well known, groups of individual muscles may be 

 caused to contract simultaneously, while some of the muscles 

 which enter into the formation of these groups may be re- 

 placed by others. This is well exemplified by the mechanism 

 of piano-playing: the index, thumb, and little finger, to form 

 one chord; the annular and thumb to form the next, etc. Trac- 

 ing this mechanism back to the structure which incites and 

 governs the muscular adduction and abduction through which 

 the keys are struck and released, we are brought back to a 

 neuron. But how is the neuron incited to activity? In other 

 words, how is the increased speed of blood through its myelin- 

 lined dendrites, cell-body, and axon incited and governed? Can 

 we ascribe this all-important function to the posterior pitui- 

 tary body? We have seen that removal of the hemispheres did 

 not prevent muscular action; a frog can swim, a pigeon fly, 

 etc., and, indeed, continue to live a considerable time months 

 if carefully fed, notwithstanding the absence of its hemi- 

 spheres. This suggests that, while the posterior pituitary 

 body either directly or indirectly incites and governs the 

 functional activity of all organs, exception should be made of 

 the brain, though it governs the circulation of this organ. 



Still, the cellular elements of all organs are supplied with 

 a net-work of terminal nerves, and it is through the inter- 

 mediary in this net-work that its functional metabolism is 

 governed by the posterior pituitary. How is the same func- 

 tion fulfilled in the hemispheres: i.e., how are its cells, the 

 neurons, "incited and governed"? Are they also supplied with 



