152 HANDBOOK OF PHYSIOLOGY. 



by the vasoconstrictor 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. 

 (/?.) Indirectly, by impressions descending from the cere- 

 brum 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 sen- 



. sory nerves, the sensory impulse being interpreted 



as an indication for diminished blood-pressure. 

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

 rectly, 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 probable 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 posi- 

 tively 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 ac- 

 cumulation 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 pres- 

 sure continues to rise gradually ; but if the amount exceed this propor- 

 tion, the rise does not continue. In this case therefore, as in the oppo- 

 site when blood is abstracted, the vaso-motor apparatus must counteract 

 the great increase of pressure, but now by dilating the small vessels, and 

 so diminishing the peripheral resistance, for after each rise there is 



