626 CIRCULATION OF BLOOD AND LYMPH. 



the existence of a vasomotor regulation of the brain circulation is 

 not conclusive. If vasohiotors are present it is possible that they 

 may serve to control the distribution of blood within the cerebral 

 area, while the general supply to the brain as a whole is increased or 

 decreased by a mechanism of another sort described 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 Wilhs, 

 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, especially when it takes 

 the form of emotional excitement. 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, di- 

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

 more blood through it. It is difficult to understand why psychical 

 activity is associated in this way with a peripheral constriction, 

 and usually an increased heart-rate, 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 im- 

 portance of this reflex in providing a greater flow to the central 

 organ at the time that it is in activity may still be admitted. A 

 general irrigation, so to speak, is provided for by this means. 

 Local vasomotors may be used to divert this flow mainly through 

 one or another cerebral area. 



Vasomotor Nerves of the Head Region. — The vasomotor 

 supply of the various parts of the head, including the mouth cavity, 

 has been investigated by many observers. It would appear from 

 the results of most of these investigations that the vasoconstrictor 

 supply for the skin, including the ears, the eye, the mouth, and buc- 

 cal glands, is derived mainly, if not entirely, from the sympathetic 

 autonomic system. These fibers arise from the spinal cord in the 

 upper thoracic nerves, first to the fifth or sixth, emerge by the rami 

 communicantes to the sympathetic chain, in which they pass up- 



