248 HANDBOOK OF PHYSIOLOGY. 



tension in the blood-vessels; as, for example, that the heart cannot, 

 with sufficient ease, propel blood into the already too full or too tense 

 arteries. 



The action of the depressor nerve in causing an inhibition of the 

 vaso-motor centre illustrates the more unusual effect of afferent impulses. 

 As a rule, the stimulation of the central end of an afferent nerve pro- 

 duces a reverse or pressor effect, and increases the tonic influence of 

 the centre, and by causing constriction of the arterioles, either locally or 

 generally, raises the blood-pressure. Thus the effect of stimulating an 

 afferent nerve may be either to dilate or to constrict the arteries. Stim- 

 ulation of an afferent nerve too may produce a kind of paradoxical effect, 

 causing general vascular constriction and so general increase of blood- 

 pressure, but at the same time local dilatation which must evidently have 

 an immense influence in increasing the flow of blood through the part. 



Course of the Vaso-motor Nerves. The cell bodies forming 

 the bulbar vaso-motor centre give off neuraxous (axis-cylinder processes), 

 some of which go to the nuclei of certain cranial nerves, while others 

 pass down the cord to end at different levels in contact with cells prob- 

 ably small cells in the anterior horn and lateral part of the gray matter. 

 These cells constitute the spinal centres. The neuraxons of the spinal 

 cells leave the cord in certain cranial nerves and in the anterior roots, 

 and end in sympathetic ganglia in contact with their cell bodies. From 

 these latter, neuraxons pass uninterruptedly to their termination in the 

 vessel wall. 



Besides the regulation of the heart beat and of the peripheral resist- 

 ance, it must be recollected that other circumstances may affect the blood 

 pressure, of which changes in the blood are the chief. First of all 

 a. As regards quantity. At first sight it w 7 ould appear probable that 

 one of the easiest ways to diminish the blood-pressure would be to re- 

 move blood from the vessels by bleeding. It has been fonnd by experi- 

 ment, however, although the blood-pressure sinks while large abstractions 

 of blood are taking place, that 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 do- 

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

 arterial system. This is due to the stimulation of the vaso-motor cen- 

 tre 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 



