70 ELEMENTARY PHYSIOLOGY less. 



of it which we shall describe in a later Lesson (XI.) as 

 the spinal bulb or medulla oblongata. Here then 

 the vaso-constrictor impulses are generated, and since they 

 are the chief agents in determining the state of contraction 

 or relaxation of the arteines of the body as a whole, this 

 definitely localisefj part of the bulb has received the 

 name of the vaso-motor centre. (Fig. 22, V.M.C.). 



The cause of the phenomenon of arterial " tone " now 

 becomes quite clear. The vaso-motor centre continually 

 generates and sends out impulses to every part, or rather 

 to very many parts, of the body, which sufKce to keep 

 the muscle fibres of the arteries sui)plying those parts in 

 a condition of slight contraction. When the impulses are 

 increased to any part, the supply of blood to that part is 

 lessened ; when the impulses are lessened the supply is 

 increased. 



But if the vaso-motor centre is to be of use it must itself 

 be under the influence of impulses which can be made 

 to play uj)on it in such a way as to determine those 

 variations in its activity which are essential to its adapt- 

 ing itself to the varying needs of either the body as a 

 whole or any small \rdrt of the body. These impulses 

 whicli govern the vaso-motor centre pass into it either 

 down from the brain above, or up from the spinal cord 

 below. As an instance of the former case we may refer 

 once again to "blushing." Here the emotion which leads 

 to the blush, starts impulses in the brain which then pass 

 down to the vaso-motor centre and modify its activity so as 

 to lessen the intensity of the impulses it sends to the blood- 

 vessels of the cheeks. As an instance of the second case 

 we may refer to the efi'ects of heat and cold ap))lied to the 

 body, as determining those variations of blood-sup] )ly to 

 the skin by wliich the temperature of the body is 

 so largely regulated (see Lesson V. ). Here the impulses 

 are started in the skin and, travelling along certain 

 sensory nerves, enter the spinal cord, pass up to the 

 vaso-motor centre, and as Ijefore lead to the necessary 

 changes in its activity. 



