72 A Manual of Veterinary Physiology. 



Experiment has shown that the vaso-motor nerves over 

 the whole body are under the control of a vaso-motor 

 centre in the medulla ; certain centres exist in the spinal 

 •cord, but these are subordinate to the centre in the 

 medulla. By means of impulses passing out from this 

 centre, the whole of the vessels of the body are kept 

 under control, dilated or contracted as needful, and in 

 this way vascular tone is maintained. 



If the vessels of a part be cut off from the vaso-motor 

 centre in the medulla, relaxation occurs through paralysis 

 of their muscular coat, and dilatation follows. 



The vaso-motor centre in the medulla is situated in the 

 floor of the fourth ventricle ; it is readily affected by the 

 quality and character of the blood circulating through it 

 — for example, an increased venous condition of the blood 

 leads to contraction of the smaller arterioles ; this con- 

 striction means a considerable opposition to the flow 

 through the arteries, as the result of which the blood 

 pressure is raised. The rise and fall in blood pressure is 

 largely under the control of this centre, through the 

 resistance or otherwise offered to the flow of blood through 

 the periphery. Stimulation of the vaso-motor centre 

 means a constriction of the smaller vessels, but it is pos- 

 sible for the opposite condition to be brought about, viz., 

 inhibition of the centre, in which case vascular dilatation 

 occurs. This is more brought home to us clinically than 

 physiologically. The engorgement of the spleen, portal 

 system, etc., seen in certain diseases, must largely be due 

 to inhibition of the vaso-motor centre, and a foil in blood 

 pressure due to the dilatation of the vessels. 



In the rabbit this can be experimentally produced 

 through a nerve termed the depressor, which is in con- 

 nection with the pneumogastric, and runs from the heart ; 

 if the central end of this nerve be stimulated, though the 

 heart-beats remain unchanged, the blood pressure slowly 

 falls, owing to the dilatation of the abdominal vessels due 

 to the inhibition of the vaso-motor centre in the medulla. 



Besides the vaso-motor centre in the brain, and the subsi- 



