THE INHIBITORY NERVES 



203 



The center is also influenced by afferent impulses which may reach it 

 from the heart itself, by the depressor nerve, or from other parts of the body. 

 These reflex stimulations of the heart through the vagus center are constantly 

 occurring during our daily life and are the most potent factors in co-ordi- 

 nations going on between the heart and the rest of the body. 



Di-s 



FIG. 182. Diagrammatic Representation of the Origin and Course of the Cardiac 

 Nerves in the Dog. Di, D$, first to fifth dorsal spinal nerves. Inhibitory fibers in blue, 

 accelerators in red. (Modified from Moret.) 



Rhythmical alterations of the heart rate occur in association with the 

 effects of the mechanical variations of pressure of the thorax on the heart 

 and blood vessels. Apparently the cardiac-inhibitory center is stimulated 

 during the fall of blood pressure. The activity of the center produces a 

 slower rate of the heart during expiration, shown in figure 243. This vari- 

 ation in heart rate disappears when the vagi are cut off from the center. 



