THE VASOMOTOR CENTRE 69 



saliva, or when the stomacli is preparing to digest food, 

 in each case the small arteries of the muscle, salivary 

 gland or stomach, dilate and so flush the part with blood. 

 The organ in fact blushes ; and this inner unseen blush- 

 ing may, like the ordinary blushing described above, 

 be brought a})out by vaso-motor nerves. It may also be 

 brought about by chemical bodies produced in the organs, 

 which, acting on the muscular walls of their arteries, 

 cause them to relax. We shall see later on that the 

 temperature of the body is largely regulated by the 

 supply of blood sent to the skin to be cooled, and this 

 supply is in turn regulated by the vaso-motor nen-ous 

 system. Indeed everywhere all over the body, the ner- 

 vous system by its vaso-motor nerves is continually super- 

 vising and regulating the supply of blood, sending now 

 more now less blood, to this or that part ; and many 

 diseases, such as those when exposure to cold causes con- 

 gestion or inflammation, are due to, or at least associated 

 with, a disorder or failure of this vaso-motor activity. 



15. The Vaso-motor Centre.— The vaso-constrictor 

 nerves, which, by causing the varying contraction in the 

 muscular walls of the arteries, thus control the supply of 

 blood to each region of the body, can all be traced back 

 to the spinal cord. They make their exit from this part 

 of the central nervpus system by the anterior roots of the 

 spinal nerves of the middle part of the cord, and after 

 passing through the ganglia of the sympathetic system 

 (see Lesson XI.) are distributed to their various 

 destinations. The impulses which these nerves convey to 

 the blood-vessels are of course received by them from the 

 spinal cord. This being the ca.se the interesting question 

 arises as to where these impulses are generated before 

 their exit from the cord. Experiment shoAvs that under 

 ordinary circumstances they come down the cord from a 

 point higher up, i.e. nearer the brain, than that at which 

 the nerves themselves pass off from the cord. In fact it 

 has been shown that they originate in a very limited 

 portion of the central nervous system, located in that part' 



