580 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 Frangois-Franck has 

 shown that the sympathetic in the neck contains afferent fibers 

 which give such reflexes.* In experiments made upon an 

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

 Wiggers f states that addition of adrenalin caused a diminution 

 in the outflow from the organ, thus showing that the adrenalin had 

 caused a constriction somewhere in the circuit. If, as some authors 

 believe, adrenalin acts only on plain muscle that is innervated by 

 sympathetic nerve fibers this result furnishes indirect evidence 

 for the existence of such fibers in the case of the brain vessels. As 

 another argument in favor of the presence of vasomotor fibers it 

 may also be mentioned that a number of observers Gulland, 

 Huber, Hunter J have demonstrated that the vessels of the brain 

 are provided with perivascular nerve plexuses. It must be ad- 

 mitted, however, that this histological fact is not satisfactory 

 unless it is supplemented by experimental evidence. Judged 

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

 of the existence of cerebral vasomotors, and, assuming that there 

 are no such fibers, 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. 



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

 Wiggers, 'American Journal of Physiology/' 1905, xiv., 

 See Hunter, "Journal of Physiologic/' 26, 465, 1902. 



