BLOOD PRESSURE 135 



which are called vasoconstrictor because they constrict the lumen of the 

 blood vessels, are transmitted to the blood vessels in the abdomen. 

 The vessels are under the constant influence of these impulses so that, 

 when the nerves that transmit them are severed, the vessels dilate and 

 thus offer less resistance to the movement of blood. The result 

 produced on the mean arterial blood pressure by cutting the two 

 splanchnic nerves is therefore a marked and sudden fall, which is im- 

 mediately recovered from if the peripheral end of one of the cut nerves is 

 stimulated artificially (Fig. 29). In choosing this experiment to prove the 

 relationship between peripheral resistance and the mean arterial blood 



Fig. 27. Effect of cutting the vagus nerve on the arterial blood pressure. 



pressure, it must be remembered that it is not entirely conclusive, since 

 the results observed on the mean arterial blood pressure from cutting 

 or stimulating the nerve may be in part explained as due to variation 

 in the total capacity of the circulation ; more room is created by cutting 

 the nerves, less room by stimulating them. 



3. The Amount of Blood in the Body 



This can be altered by hemorrhage or transfusion, and the results 

 of such procedures are of interest not only on account of their physi- 

 ological bearing, but also because of their great practical importance. 



