CIRCULATION 



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But it has been found that, if, after section of the spinal 

 cord high up, the animal be kept alive for some days, the 

 dilated arterioles again contract, and stimulation of afferent 

 nerves entering the cord below the point of section causes 

 a further constriction. If now another section be made 

 further down the cord, the arterioles supplied by nerves 

 coming from below the point of section will again dilate. 

 This shows that secondary vaso-eonstrietor centres, tonic in 

 action and capable of having their activity reflexly increased, 

 exist all down the grey matter of the spinal cord. Normally 

 these are under the domain of the dominant centre, but 

 when this is out of action they then come into play. 



B. Vaso - dilator Centre. (a) Mode of Action. This 

 mechanism is not constantly in action. Section of a vaso- 

 dilator nerve does not cause vascular dilatation. It may be 

 excited reflexly, but in a different manner from the vaso- 

 constrictor mechanism. 



Stimulation of an afferent nerve causes a dilatation of the 

 arterioles in the part from which it comes, and a constric- 

 tion of the arterioles throughout the rest of the body. If 

 a sapid substance, such as pepper, be put in the mouth, 

 the buccal mucous membrane and the salivary glands become 

 engorged, while there is a constriction of the arterioles 

 throughout the body. The vaso-dilator central mechanism 

 is not general in its action like the vaso-constrictor, but is 

 specially related to the different parts of the body. 



This is a matter of the greatest importance in physiology 

 and pathology. It explains the increased vascularity of a 

 part when active growth is going on. The changes in the part, 

 or the products of these, stimulate the afferent nerve. This 

 reflexly stimulates the vaso-dilator mechanism of the part, 

 and thus causes a free flow of blood into the capillaries, and 

 at the same time maintains or actually raises the arterial 

 pressure by causing a general constriction of the arterioles, 

 and thus forces more blood to the situation in which it is 

 required. 



The same process occurs in the case of the stomach during 

 digestion, in the case of the kidney during secretion, and in 

 the process of inflammation. 



Not only does peripheral stimulation act in this way, but 



