THE VASCULAR MECHANISM. 235 



these two factors is seen in the form of the blood-pressure tracing, 

 which often shows, as it rises, a small notch or step (fig. 92) ; the first 

 part of the rise is due to the direct action of the splanchnic nerve ; the 

 rise above the notch is due to adrenalin. Owing to the setting free of 

 adrenalin, stimulation of a splanchnic nerve causes diminution in the 

 volume of the limbs. These effects are produced not only when the 

 splanchnic nerve is divided and its peripheral end is directly stimu- 

 lated, but also when the impulse passing along the splanchnic nerve 

 originates in the vaso-motor centre itself, as in asphyxia. After 

 extirpation of the suprarenal glands, adrenalin can no longer be set 

 free into the blood stream, and stimulation of the splanchnic nerve 

 causes a much smaller rise of blood pressure ; the vaso-constriction is 

 limited to the abdominal vessels, and the blood-vessels of the limbs are 

 passively dilated by the higher arterial pressure (fig. 93). 



FIG. 92. Blood-pressure tracing, showing effect of stimulation of left 

 splanchnic nerve, (von Anrep. ) 



Shock. After severe injuries or profuse haemorrhage an individual 

 may pass into the condition known as shock. The characteristic 

 symptoms of shock are a low arterial blood pressure, and disappearance 

 of many of the normal reflexes ; the pulse is rapid and feeble, the 

 respiration is shallow, and the temperature is low. A similar condition 

 sometimes occurs after surgical operations, and can be experimentally 

 produced in animals. Its causation is not understood, though various 

 explanations have been offered to account for the low blood pressure, 

 which forms such an important part of shock. The low blood pressure 

 is not due to paralysis of the vaso-motor centre, since the centre 

 responds to afferent stimuli almost as readily as in the normal animal. 

 It has been suggested that in shock the veins dilate and that the 

 blood accumulates in them, leaving the arterial system comparatively 

 empty : in these circumstances the arterial pressure will be low, and 

 the nutrition of the brain will be impaired. Other explanations have 

 also been offered, none of which is entirely satisfactory. 



