THE CIRCULATION OF THE BLOOD 353 



in the rate or force of the heart, and this is especially the case if the pressure 

 has been above the normal. 



The decrease in the peripheral resistance, i.e., in the arteriole contraction 

 and its effect on the blood-pressure may be brought by a depression or in- 

 hibition of the activity of the general vaso-motor center due to the arrival 

 of nerve impulses transmitted to it by certain afferent nerves, e.g., the de- 

 pressor nerve (see page 386), when, after division, its central end is stimu- 

 lated; or by the inhalation of amyl nitrite, which it is generally believed, 

 acts locally on the muscle walls of the arterioles and causes their relaxation 

 (Fig. 164). 



3. When both the force of the heart and .the peripheral resistance are sim- 

 ultaneously diminished, there will be a rapid fall in pressure. The former 

 factor tends to decrease, the latter factor to increase the velocity of 'outflow. 



FIG 164 TRACING SHOWING THE FALL OF BLOOD-PRESSURE IN THE CAT, following the 

 inhalation 01 amyl nitrite. Inhalation begun at X. 



According as the one or the other preponderates i will-there be a decrease or 

 an increase in velocity. If they balance each other there will be no change 

 This condition is also a pathologic rather than a physiologic condition and 

 observed in states of profound depression due to serious injuries. 



Local Variations in the Arterial Blood-supply .-The ^variations i 

 pressure and velocity from variations either in the activity of the heart or in 

 ^peripheral resistance recorded in preceding ^^ffi^hS 

 the arterial system in its entirety; but it is f^^j^^TlK? 



^^1^^^^^^^^^ 



