;i62 A MANUAL OF PHYSIOLOGY 



The kind of stimulus seems to have something to do with 

 the direction of the reflex vaso-motor change, for while 

 electrical stimulation of every muscular nerve, even of the 

 very finest twigs that can be isolated and laid on electrodes, 

 provokes always, whether the shocks follow each other 

 rapidly or slowly, a rise of general blood-pressure, mechanical 

 stimulation of a muscle, as by kneading or massage, causes 

 a fall. The condition of the afferent fibres also exerts an 

 influence. For example, excitation of the central end of a 

 sciatic nerve that has been cooled is followed by vaso- 

 dilatation and fall of pressure, the opposite of the ordinary 

 result. These and similar facts have led to the idea that 

 most afferent nerves contain two kinds of fibres, whose 

 stimulation can affect the activity of the vaso-motor centres, 

 * reflex vaso-constrictor,' or ' pressor ' fibres, and ' reflex 

 vaso-dilator,' or * depressor ' fibres. The branch of the 

 vagus, however, to which the name ' depressor ' has been 

 specially given, is the only peripheral nerve the excitation of 

 which is in all circumstances followed by a general diminu- 

 tion of arterial pressure. If specific ' depressor ' fibres exist 

 elsewhere, they are so mingled with ' pressor ' fibres that 

 their action is masked when both are stimulated together. 

 The state of the vaso-motor centre is a third factor, which 

 has some importance in determining the result of reflex 

 vaso-motor stimulation. For instance, in an animal deeply 

 anaesthetized with chloroform or chloral, excitation of an 

 ordinary sensory nerve may cause, not a rise, but a fall of 

 blood-pressure. 



An interesting illustration of the reciprocal relation 

 between different parts is found in the opposite behaviour of 

 the vessels of the skin and those of the internal organs, 

 which is often observed during reflex stimulation of the 

 vaso-motor centres. For example, stimulation of the cut end 

 of the sciatic causes, as we have already seen, a notable rise 

 in the blood-pressure and extensive vaso-constriction. This 

 certainly involves the splanchnic area ; but superficial parts, 

 as the lips, may be seen to be flushed with blood. In 

 asphyxia, when the vaso-motor centres are directly stimu- 

 lated by the venous blood, this antagonism is still better 



