VASOMOTOR SUPPLY OF THE ORGANS. 625 



against vasomotors are based upon a determination of the amount 

 of flow through the brain or upon measurements of pressure within 

 the circle of WilUs, it has been shown that an undetermined fac- 

 tor is involved which makes such observations unsatisfactory. 

 It has been shown * in experiments upon dogs that when the intra- 

 cranial pressure is raised so high as to obliterate the circulation 

 through the brain substance itself an abundant circulation may be 

 maintained through the skull by perfusion into the internal carotid 

 — that is to say, there are paths between the circle of Willis and the 

 emergent veins other than the capillary circulation through the 

 brain substance. One such path is furnished by an anastomosis 

 at the base of the skull between the circle (through the internal 

 carotid) and the ophthalmic branch of the internal maxillary artery. 

 It might, therefore, very well happen that the circulation in the brain 

 substance may be changed without materially affecting the amount 

 of blood-flow from the brain, owing to the fact that these other 

 paths are open. Weber, who used the plethysmographic method 

 of measuring the volume of the brain, states positively that stimu- 

 lation of the cervical sympathetics, of the cortical surface, and of 

 various sensory nerves gives in animals such changes in brain 

 volume as can only be interpreted by the assumption that the brain 

 vessels possess both vasoconstrictor and vasodilator nerve-fibers. 

 Since these reactions can be obtained reflexly after destruction of 

 the general vasomotor center in the medulla, he is forced to assume 

 a special vasomotor center for the brain lying further forward than 

 the medulla, a conclusion which is not entirely satisfactory. 

 Plethysmographic observations on the brain during sleep, ag 

 reported by some observers (p. 265), have also been inter- 

 preted as indicating the presence of a local vasomotor apparatus. 

 An argument of a different kind in favor of vasomotor fibers has 

 been submitted by Wiggers.f In experiments made upon an iso- 

 lated brain (in the skull) perfused with an artificial circulation, he 

 states that addition of epinephrin caused a diminution in the out- 

 flow from the organ, thus showing that the epinephrin had caused a 

 constriction somewhere in the circuit. If, as some authors believe, 

 epinephrin acts only on plain muscle that is innervated by sympa- 

 thetic nerve-fibers, this result furnishes indirect evidence for the ex- 

 istence of such fibers in the case of the brain vessels. Using the 

 same method, this author states that electrical stimulation applied 

 directly to the sheath of the internal carotid at its entrance into 

 the skull also causes a decrease in the outflow, a fact which would 

 indicate the existence of constrictor fibers running in the sheath 

 •of this artery. On the whole, it will be seen that the evidence for 



* Eyster, Burrows, and Essick, "Journal of Exp. Medicine," ii, 489, 1909. 

 t Wiggers, "American Journal of Physiology," 14, 452, 1905; and 21, 454, 

 1908. 



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