178 UNCONSCIOUS NERVOUS OPERATIONS 



spinal cord communicate with the brain, they are all con- 

 nected with the brain also. Certain fibers of the vagus 

 proper carry the afferent influences from the heart to the 

 brain, and by them the brain is kept informed of the 

 heart's condition. Fibers of the spinal accessory and 

 the sympathetic nerves convey motor impulses from the 

 brain to the heart. 



243. Accelerator Fibers of Cardiac Nerves. Experiments 

 have proved that it is the fibers from the sympathetic 

 ganglia which convey to the heart the impulse which 

 accelerates and strengthens its beat. They are supposed 

 to run up the spinal cord to an accelerator center in the 

 medulla oblongata (Fig. 99). 



244. Inhibitory Fibers of Cardiac Nerves. Stimulation of 

 the fibers of the motor branch of the vagus running to the 

 heart not only does not increase its action, but retards or 

 inhibits it, and may entirely stop its beating. By this 

 means the brain keeps a continual check upon the action 

 of the heart. It has been found that if the vagus nerves 

 of a dog are divided, the beat of the heart is quickened. 



245. Reflex Inhibition. A violent blow upon the stom- 

 ach or abdomen, as is well known, may cause fainting, due 

 to stoppage of the heart's action. The stimulus of the 

 blow is carried by the sympathetic nerves of the epigas- 

 tric plexus, situated around the pit of the stomach, to the 

 thoracic ganglia and thence to the medulla oblongata. 

 There the impulse is reflected along the efferent fibers of 

 the vagus to the heart muscles^ and their action is 

 checked. Fainting may be caused in a similar way by 

 the action of severe pain or strong emotion upon the nerve 

 cells of the brain. The impulse reaches the region of the 

 medulla oblongata from which the vagus arises, and is 

 sent on to the heart. 



