SECTION XI 



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 meta- 

 bolic changes consequent on muscular activity. The muscles there- 

 fore during activity require a greater supply of blood in order to 

 obtain from it the oxygen necessary 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 sub- 

 ordination 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 may 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 sympa- 

 thetic 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 sympa- 

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

 section must have passed off shows that the ear-vessels are con- 

 tinually 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 



1103 



