354 TEXT-BOOK OF PHYSIOLOGY 



of blood discharged from the ventricle in a unit of time. As a result there 

 will be a diminution in the volume of the blood in the arteries, a lessening 

 distention and a fall of pressure, for the reason that the outflow into the 

 capillaries and veins is greater under the existing pressure than the 

 inflow from the heart. As the pressure falls the velocity diminishes. 

 This continues until the outflow no longer exceeds the inflow. Equi- 

 librium will then again be established but the pressure will be at a 

 lower level. A decrease in the rate and force of the heart-beat follows 

 an augmentation of, or an increased activity or tonus of the cardio- 

 inhibitor center. This condition may be brought about by the arrival 

 of nerve impulses (i) from the cerebrum in consequence of emotional states 

 of a depressing character (see page 326); and (2) from different regions 

 of the body, transmitted by afferent nerves, the terminals of which are 



FIG. 159. TRACING SHOWING REFLEX INHIBITION OF THE HEART AND FALL OF BLOOD- 

 PRESSURE IN THE CAT, following stimulation of the central end of the vagus nerve. Stimu- 

 lation began at S. 



stimulated by physiologic processes. In either case the nerve impulses 

 augment or excite the normal activity of the cardio-inhibitor center 

 causing it to discharge a greater number of nerve impulses through its 

 efferent fibers to the heart, thus checking or decreasing the heart rate 

 and to this extent antagonizing the influence of the cardio-accelerator 

 center. 



Though it is difficult to point out any particular region of the 

 body from which nerve impulses, developed by physiologic processes, are 

 transmitted by afferent nerves to this center and excite it to activity, never- 

 theless the results of physiologic experimentation and clinic observation 

 lead to the inference that this is the case under normal conditions. Thus 

 a partial or complete augmentation or condition of increased activity of 

 the cardio-inhibitor center as shown by a decrease in the rate of the heart 

 follows stimulation of the central end of many afferent nerves ; thus, stimu- 

 lation of the central end of the divided vagus, the opposite nerve being 

 intact, causes not only a decrease in the rate of the heart, but a decrease 

 in the blood pressure (Fig. 159). Stimulation of the infra-orbital nerve, the 

 dorsal nerve of the foot, the splanchnic, etc., have a similar effect. Patho- 

 logic processes and operative procedures in different regions of the body 



