3S 6 TEXT-BOOK OF PHYSIOLOGY 



inhalation of amyl nitrite as shown by the fall of the blood-pressure (Fig. 

 161). This agent, it is generally believed, exerts its action on the muscle 

 fiber. With the oxidation of the amyl nitrite the arteriole muscles contract 

 and the pressure soon rises to the normal level. Other nitrites have a 

 much more enduring effect. 



'A rise in blood-pressure much above the normal and due to increased 

 peripheral resistance from whatever cause, leads to a decrease in the rate 

 of the heart in consequence of an increase in the tonus or activity of 

 the cardio-inhibitor center. The increased tonus of this center is 

 caused by the mechanical stimulation of the peripheral terminations 

 of the depressor nerve in the walls of the root of the aorta. The 

 distention of the aorta is believed to be the adequate stimulus. With 

 the diminution in the rate and force of the heart the volume discharged 

 into the arteries in a unit of time is lessened, and a rise in the pressure 



FIG. 161. TRACING SHOWING THE FALL OF BLOOD-PRESSURE IN THE CAT, following the 

 inhalation of amyl nitrite. Inhalation begun at X. 



sufficiently high to seriously embarrass the action of the heart is to a 

 considerable extent prevented. By this retardation of the rate of the 

 heart a limit is set to the rise of pressure, the result of arteriole contraction. 

 That the depressor nerve is the agent that mediates this result is indicated 

 by the fact that if the vagus nerves are divided, the retarding effect is not 

 manifested. The high blood-pressure may be removed by agents which 

 overcome peripheral resistance. With the fall of the pressure, the heart 

 rate increases. Therefore it may be stated that a rise in pressure is 

 attended by a decreased heart rate, while a fall of pressure is attended by 

 an increased heart rate. 



A fall in blood-pressure more or less continuous and due to decreased 

 peripheral resistance from whatever cause, leads to an increase in the 

 rate of the heart in consequence of the loss of tone or tonic activity of the 

 cardio-inhibitor center, the result of anemia of the nerve centers in general. 

 The force of the heart is at the same time diminished and the volume of 

 the blood discharged into the arteries is thereby lessened. The low pres- 



