236 ESSENTIALS OF PHYSIOLOGY. 



Local Changes in the Arterioles. The variations in the activity of 

 the vaso-motor centre, brought about by the means already described, 

 are chiefly directed to regulating the mean arterial pressure and to- 

 providing an efficient supply of blood to the brain. Alterations in the 

 calibre of the arterioles in any one organ of the body as distinct from 

 the body in general are sometimes due to the presence of vaso-dilator 

 nerves, as, for example, the chorda tympani. There is, however, another 

 factor of great importance. Generally speaking, increased functional 

 activity of any organ of the body is accompanied by dilatation of its. 

 arterioles and an increased flow of blood through it; this is brought 



FIG. 93. Stimulation of a splanchnic nerve after removal of the suprarenal glands. 



(von Anrep. ) 

 L.V., volume of leg enclosed in a plethysmograph ; B.P., arterial blood pressure. 



about partly, or even wholly, by the direct action upon the walls of 

 the arterioles of the waste products (metabolites) formed by the organ, 

 during its activity. They include carbonic acid, lactic acid, and 

 probably other substances, and experiment has shown that when these 

 products are added to the blood passing through an organ, e.g. the 

 heart or skeletal muscle, its arterioles dilate. This local mechanism 

 provides a means by which the increased demands of a tissue for 

 nutritive material and oxygen, when it is active, are met by an increase 

 in the amount of blood passing through it. 



THE CEREBRAL CIRCULATION. 



The circulation of the blood through the brain is peculiar in two 

 respects. In the first place, the brain is enclosed in a rigid case, the 

 skull, which it almost completely fills, and, secondly, there is at present 

 no conclusive evidence that the arterioles of the brain are under the 



