CIRCULATION" OF THE BLOOD. 157 



(2.) By stimulation of the centres in spinal cord. 



Possibly directly or indirectly, certainly reflexly. 

 (3.) By stimulation of the local centres for each vascular area, by 



the vaso-constrictor nerves, or directly by means of altered 



blood. 



B. A decrease of the blood pressure may be produced: 



(1.) By stimulation of the vaso-motor centre in medulla, either 

 (a.} Directly, as by oxygenated or aerated blood. 

 (p. ) Indirectly, by impressions descending from the cerebrum 



e.g., in blushing. 



(y.) Reflexly, by stimulation of the depressor nerve, and 

 consequent dilatation of vessels of splanchnic area, and 

 possibly by stimulation of other sensory nerves, the sen- 

 sory impulse being interpreted as an indication for 

 diminished blood-pressure. 

 (2.) By stimulation of the centres in spinal cord. Possibly 



directly, indirectly, or reflexly. 



(3.) By stimulation of local centres for each vascular area by the 

 vaso-dilator nerve, or directly by means of altered blood. 



4. Changes in the blood. a. As regards quantity. At first sight it 

 would appear that one of the easiest ways to diminish the blood-pressure 

 would be to remove blood from the vessels by bleeding; it has been found 

 by experiment, however, that although the blood-pressure sinks whilst 

 large abstractions of blood are taking place, as soon as the bleeding ceases 

 it rises rapidly, and speedily becomes normal; that is to say, unless so 

 large an amount of blood has been taken as to be positively dangerous to 

 life, abstraction of blood has little effect upon the blood-pressure. The 

 rapid return to the normal pressure is due not so much to the withdrawal 

 of lymph and other fluids from the body into the blood, as was formerly 

 supposed, as to the regulation of the peripheral resistance by the vaso- 

 motor nerves; in other words, the small arteries contract, and in so doing 

 maintain pressure on the blood and favor its accumulation in the arterial 

 system. This is due to the stimulation of the vaso-motor centre from 

 diminution of the supply of blood, and therefore of oxygen. The failure 

 of the blood-pressure to return to normal in the too great abstraction 

 must be taken to indicate a condition of exhaustion of the centre, and 

 consequently of want of regulation of the peripheral resistance. In the 

 same way it might be thought that injection of blood into the already 

 pretty full vessels would be at once followed by rise in the blood-pressure, 

 and this is indeed the case up to a certain point the pressure does rise, 

 but there is a limit to the rise. Until the amount of blood injected 

 equals about 2 to 3 per cent, of the body weight the pressure continues to 

 rise gradually; but if the amount exceed this proportion, the rise does not 

 continue. In this case therefore, as in the opposite when blood is ab- 



