THE CIRCULATION OF THE BLOOD 



353 



impulses from the cerebrum in consequence of emotional states such as 

 fright or fear, and from different regions of the body transmitted by 

 afferent nerves e.g. the cutaneous nerves when stimulated by a fall of 

 temperature. Stimulation of the central end of many divided afferent 

 nerves e.g. the sciatic, increases the activity of the vaso-motor center as 

 shown by a marked rise of the blood pressure (Fig. 157). With the cessa- 



FIG. 157. A TRACING SHOWING AN INCREASE IN THE BLOOD-PRESSURE IN THE CAROTID 

 ARTERY or A RABBIT DUE TO AN INCREASE IN THE PERIPHERAL RESISTANCE FROM A CONTRACTION 

 OF THE ARTERJOLES CAUSED BY REFLEX STIMULATION OF THE VASO-MOTOR CENTER. The nerve 

 stimulated was the sciatic. Stimulation began at s. The rate of the heart-beat is unchanged. 

 With the cessation of the stimulation the blood-pressure falls for the reverse reasons. 



tion of the stimulation from whatever cause the center returns to its 

 former degree of activity, the arterioles again relax and the pressure falls 

 to the normal level. 



Direct stimulation, and contraction of the arteriole muscles, follows 

 the injection into the blood of minute amounts of adrenalin as shown by the 

 rise of the blood pressure (Fig. 158). This- agent, it is generally believed, 



FIG. 158. TRACING SHOWING THE RISE OF BLOOD PRESSURE IN A CAT after the intra- 

 venous injection of minute doses of adrenalin. Injection was begun at x. The abscissa 

 should be 40 millimeters lower. 



exerts its action on the receptor substance at the myo-neural junction. 

 With the oxidation of the adrenalin, the arteriole muscles relax and the 

 pressure falls to the normal level. 



The converse of the foregoing statements also in large measure holds 

 true. 



i. A decrease in the rate and force of the heart when the blood-pres- 

 sure is in a condition of equilibrium will lead to a decrease in the amount 

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