222 



ESSENTIALS OF PHYSIOLOGY. 



described as its tonic inhibitory action, and is particularly well marked 

 in the dog and horse. On section of the vagus nerves the tonic action 

 is abolished and the heart beats more rapidly. The tone of the vagus 



FIG. 84. Upper tracing shows acceleration of the heart due to 

 stimulation of an accelerator nerve. 



centre can be reflexly increased or diminished by afferent impulses 

 reaching it from various parts of the body. The most important 

 afferent paths are (1) the depressor nerve, (2) afferent fibres running in 



FIG. 85. Reflex slowing of the heart due to stimulation of central end 

 of one vagus. The other vagus is intact. 



the vagus from the lungs and from the heart itself, and (3) many 

 sensory nerves. 



(1) The depressor nerve is purely afferent, and, starting in the 

 walls of the aortic arch, it runs up the neck on each side, in some 

 animals (rabbits) as a separate nerve, in others bound up with the 





