THE BLOODVESSELS 87 



Since, in the above experiment, mere severance of the nerves 

 which connect the bloodvessels with the central nervous system 

 leads to a dilatation of the arterioles, it is evident that impulses 

 are, under normal conditions, being continually sent out along 

 the nerves from the vasomotor centres. These impulses keep 

 the arterioles normally in that state of medium or partial con- 

 striction which has already been described as arterial ' tone.' 

 Now, inasmuch as the function of the vasomotor nerves is to 

 regulate the blood-supply to any given area of the body, in exact 

 accordance with the varying needs of that area, ' tone ' becomes 

 a factor of the utmost importance in this regulative mechanism. 

 Without it all the arteries of the body would, in the ordinary 

 passive condition of rest, be dilated to their full extent ; hence 

 no increased supply of blood could be provided except by an 

 augmented activity of the heart, which would, of course, affect 

 the body as a whole, and not any one limited part of it. ' Tone ' 

 insures that an arteriole may both dilate and contract, according 

 as it receives less or more of the continuous constricting im- 

 pulses, and thus the regulation of a varying blood-supply is made 

 extremely perfect. 



If the sciatic or brachial nerve be divided in the dog, as a rule 

 the constrictor influence over the bloodvessels of the limb is 

 lost, the foot-pads flush, and the feet rise in temperature. If 

 the central end of the divided sciatic be stimulated, it is generally 

 followed by a contraction of the bloodvessels and a rise in 

 pressure. Occasionally the tone of the centre in the medulla 

 is not raised but reduced, and dilatation of the vessels and a 

 fall in blood-pressure results. This experiment suggests that 

 most afferent nerves such as the sciatic carry both pressor and 

 depressor fibres, and that the effects which follow experimental 

 stimulation depend upon whichever set of fibres is most 

 efficiently stimulated (Fig. 38). 



The best example of a depressor nerve is one we have already 

 studied under that name (p. 57) ; it is the only peripheral nerve 

 the stimulation of which invariably reduces blood-pressure. 

 This nerve, it will be remembered, is capable of regulating the 

 work of the heart by taking off strain through the medium of 

 the abdominal venous cistern. Stimulation of the depressor 

 acts like division of the splanchnic— viz., the abdominal vessels 

 fill with blood and the blood-pressure falls. 



In spite of the enormous importance of the vasomotor centre 

 in the medulla, experimental inquiry shows that if the cord be 

 divided in the lumbar region the vessels of the hind-limb dilate 

 and the blood-pressure falls ; but if the animal be kept alive 

 the blood-pressure probably returns to the normal, though it is 

 once again lost by destroying the already divided cord. These 



