560 CIRCULATION OF BLOOD AND LYMPH. 



hand, have obtained results, especially from stimulation of the 

 cervical sympathetic, which indicated local vasoconstriction or vaso- 

 dilatation in the brain. It would seem, however, that these latter 

 observers have not excluded the possibility that the variations 

 in pressure obtained by them were due to reflex effects upon the 

 blood-vessels of the body, especially as Francois-Franck has 

 shown that the sympathetic in the neck contains afferent fibers 

 which give such reflexes * As an argument in favor of the presence 

 of vasomotor fibers it may also be mentioned that a number of 

 observers Gulland, Huber, Hunter f have demonstrated that the 

 vessels of the brain are provided with perivascular nerve plexuses. 

 It must be admitted, however, that this histological fact is not con- 

 clusive unless it is supplemented by experimental evidence. Judged 

 from this latter standpoint, we have no satisfactory proof at present 

 of the existence of cerebral vasomotors, and, accepting this negative 

 evidence, we may ask by what means is the circulation in the 

 brain regulated? The simplest view is that proposed by Roy and 

 Sherrington. According to these authors, the blood-flow through 

 the brain is controlled indirectly by vasomotor effects upon the 

 rest of the body. When, for example, a vasoconstriction occurs 

 in the skin or the splanchnic area the result is a rise of pressure 

 in the aorta and therefore a rise of pressure in the circle of Willis, 

 which then forces more blood through the brain. Adopting this 

 view, we can understand the teleology of certain well-known vaso- 

 motor reflexes. Stimulation of the skin generally causes a reflex 

 constriction and rise of pressure, and one can well understand that 

 this result is valuable if it means a greater flow of blood through 

 the brain, since under the conditions of nature such stimulation, 

 especially when painful, demands alertness and increased activity 

 on the part of the animal. Attention has also been called to the 

 fact that in plethysmographic observations on man the most 

 certain and extensive constrictions of the skin vessels are those 

 caused by increased mental activity. Mosso has shown by observa- 

 tions upon men with trephine holes in the skull that the constriction 

 of the limbs is always accompanied by a dilatation of the brain. 

 This fact, therefore, fits exactly the view that is being considered. 

 The peripheral constriction, by raising general blood-pressure, dilates 

 the brain more or less, and, what is more important, drives more 

 blood through it. It is difficult to understand why psychical 

 activity is always associated in this way with a peripheral con- 

 striction unless the object of the reflex is to increase the blood- 

 supply to the brain. Even if vasomotor fibers are subsequently 

 shown to be present in the brain, the importance of this reflex in 



* Francois-Franck, "Journal de phys. et de path, gen.," 1, 724, 1889. 

 f See Hunter, " Journal of Physiologie, " 26, 465, 1902. 



