SECTION X 

 THE NERVOUS CONTROL OF THE BLOOD-VESSELS 



DURING muscular activity the metabolism of the body as a whole, as judged 

 by its gaseous interchanges, may be increased six- or eight-fold. This 

 increase is due almost exclusively to the additional metabolic changes 

 consequent on muscular activity. The muscles therefore during activity 

 require a greater supply of blood in order to obtain from it the oxygen neces- 

 sary for their contraction, and to get rid of the carbon dioxide, which is the 

 end-result of their activity. In the same way every organ of the body 

 requires an increased blood-supply during activity. .Blood must be diverted 

 from the inactive to the active tissues. All parts of the body must co- 

 operate in subordination to the activity of that tissue whose function for the 

 time being is of the greatest importance to the organism. This subordination 

 of the part to the whole, i.e. of every part to the organ whose activity is 

 specially evoked by the needs of the whole organism, is chiefly effected 

 through the central nervous system, though local and chemical mechanisms 

 also play some part in the process. 



Our knowledge of the nervous control of the blood-vessels dates from the 

 discovery by Claude Bernard that nerve fibres run in the cervical sympathetic 

 to the blood-vessels of the head and neck, and maintain them in a state of 

 tonic constriction. Bernard showed that if in the rabbit the cervical 

 sympathetic on one side be divided, the vessels in the corresponding ear 

 dilate. Vessels come into prominence which were previously invisible, and 

 on account of the greater flow of blood thus produced, the ear on the side 

 of the section becomes warmer than the normal ear. If the head end of the 

 divided sympathetic nerve be stimulated, all the vessels of the ear contract, 

 and the ear becomes colder than that of the other side. The fact that the 

 dilatation of the vessels is produced by section of the cervical sympathetic 

 and lasts for a considerable time after any irritant effect of the section 

 must have passed off shows that the ear- vessels are continually under the 

 influence of tonic constrictor impulses proceeding to them along the nerve 

 fibres of the cervical sympathetic. 



It can be easily shown that these impulses take their origin in the central 

 nervous system. The paralysis of the ear- vessels, though lessening the resist- 

 ance to the flow of blood there, affects too small a vascular area to have 

 any marked influence on the total resistance of the circulation and therefore 

 on the arterial blood-pressure. If the spinal cord be divided on a level 



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