THE ACCELERATOR NERVES 



213 



Inhibitory Reflexes. The inhibitory center is influenced by afferent 

 nerve 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 vagus center are constantly occurring during our daily life and are 

 the most potent factors in co-ordinations going on between the heart and 

 the rest of the body. 



FIG. 182. Arterial Blood Pressure of the Dog, Showing the Effect on the Heart Rate 

 of Cutting both Vagus Nerves as marked. The scale to the left shows the pressure in 

 millimeters of mercury. Time in seconds. The momentary inhibition just before the 

 nerves were cut is probably due to mechanical stimulation of the nerves. (Hill and 

 Chilton.) 



The vagus trunk itself contains afferent fibers, the depressor nerves, 

 that arise from sensory endothelia in the heart itself and in the aortic 

 arch. These endings are stimulated by excessive mechanical pressure. - 

 Their nerve impulses react on the vagal motor cells to produce reflex 

 inhibition, hence the cardiac slowing that relieves the pressure that pro- 

 duced the reflex. This reflex apparatus is one of the most interesting 

 self protecting mechanisms in the mammalian body. 



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 cardio-inhibitory center is stimulated 

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

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

 variation in heart rate disappears when the vagi are cut off from the 

 center. The variations from this cause are called sinus arrhythmia in 

 clinical literature. Such variations are purely physiological and normal 

 in character. 



The Accelerator Nerves. The influence of the accelerator nerves 

 distributed to the heart through the thoracic sympathetic, is the reverse 

 of that of the vagus. Stimulation of the sympathetic, even of one side, 

 produces acceleration of the rate of the heart-beats, augmentation of the 

 amplitude, or force, and better or at least faster conduction through the 

 nodal system according to certain observers. Section of the nerveproduces 



