THE CIRCULATION OF THE BLOOD 



355 



whereby the terminals of afferent nerves are stimulated, frequently lead to 

 a decrease in the rate of the heart and even to complete cessation, which 

 may be regarded as an expression of an augmentation or excitation, 

 more or less complete of the cardio-inhibitor center. It is for this reason 

 that atropin is frequently given before surgical operations in order to 

 prevent the passage of nerve impulses to the heart which might otherwise 

 more or less completely inhibit its action (see page 327). With the de- 

 crease in rate there is usually a marked fall of blood-pressure. 



The effect on the heart rate that follows a partial or complete excita- 

 tion of the cardio-inhibitor center is shown by stimulating the peripheral 

 end of the divided vagus nerve; at once the heart stops and the pressure 

 abruptly falls (Fig. 160). Mechanical 

 pressure of the vagus has a similar effect. 

 The intra-venous injection of muscarin 

 stimulates the myo-neural junction 

 causing the arrest of the heart and a fall 

 of blood-pressure. Any increase in the 

 activity of the cardio-inhibitor appa- 

 ratus in any part of its course will be 

 followed by a decrease in the rate and 

 force of the heart and a consequent fall 

 of the blood-pressure. 



2. A decrease in the peripheral re- 

 sistance, due to decreased contraction 

 of the arteriole muscles will by increas- 

 ing the outflow of blood lead to a de- 

 crease in the volume of blood in the 

 arteries and a fall of pressure, even 

 though the rate and force of the heart 

 remain unchanged. Decreased contrac- 

 tion of the arteriole muscles may be 

 brought about (i) by a decrease in the 

 activity of the general vaso-mo tor center, 

 and (2) by the direct action of drugs, e.g., 

 amyl nitrite. The vaso-motor center 

 may be depressed or inhibited in its 

 activity (i) by a decrease in the per- 

 centage of carbon dioxid in the blood 

 and (2) by the arrival of nerve impulses from the cerebrum in consequence 

 of emotional states of a depressing character and from different regions of 

 the body, transmitted by afferent nerves, e.g. y the cutaneous nerves when 

 stimulated by a rise of temperature. Stimulation of the central end of 

 divided afferent nerves, e.g., the " depressor nerve" (see page 390) di- 

 minishes the activity of the vaso-motor center as shown by a marked fall 

 of the blood-pressure. With the cessation of the stimulation from what- 

 ever cause the center returns to its former degree of activity, the arterioles 

 again contract and the pressure rises to the normal level. 



A similar fall of the blood-pressure is observed on stimulating the 

 central end of other divided afferent nerves. 



Direct inhibition and relaxation of the arteriole muscles follows the 



FIG. 160. A TRACING OF THE BLOOD- 

 PRESSURE IN THE CAROTID ARTERY OF A 

 RABBIT, showing a sudden decrease in 

 the pressure due to an arrest in the rate 

 and force of the heart-beat the result of 

 stimulating the vagus nerve from " on " 

 to "off." With the cessation of the stimu- 

 lation the pressure began to rise as the 

 rate and the force of the heart-beat re- 

 turned. (The abscissa should be 20 mm. 

 lower.) 



