254 VETERINARY PHYSIOLOGY 



gives the result of stimulating the former, but sometimes 

 the stimulation of the latter masks this effect. (Practical 

 Physioloyy. ) 



In the mammal three sets of nerve fibres pass to the 

 heart : 



1st. The superior cardiac branch of the vagus starts from near 

 the origin of the superior laryngeal nerve, and passes to the 

 heart to end in the endocardium (fig. 120, S.C.). 



2nd. The inferior cardiac branch of the vagus leaves the main 

 nerve near the recurrent laryngeal, and passes to join the 

 superficial cardiac plexus in the heart (fig. 120, 1.C.). 



'3rd. The sympathetic nerve fibres come from the superior 

 thoracic and inferior cervical ganglia, and also end in the 

 superficial cardiac plexus (fig. 120, $.). 



Functions of the Cardiac Nerves. A. The Superior Cardiac 

 Branch of the Vagus is an ingoing nerve. Section produces 

 no effect ; stimulation of the lower end causes no effect ; 

 stimulation of the upper end causes slowing of the heart and 

 a marked fall in the pressure of blood in the arteries, and 

 it may cause pain. The slowing of the heart is a reflex effect 

 through the inferior cardiac branch ; and the fall of blood 

 ' pressure, which is the most manifest effect, is due to a refiex 

 dilatation of the vessels of the abdomen, causing the blood 

 to accumulate there, and thus to lessen the pressure in the 

 arteries generally. On account of its effect on the blood 

 pressure, this nerve is called the depressor nerve. 



B. Inferior Cardiac Branch of Vagus. Section of the vagus 

 or of this branch causes acceleration of the action of the 

 heart. The nerve is therefore constantly in action. Stimula- 

 tion of its central end has no effect ; stimulation of its 

 peripheral end causes a slowing or stoppage of the heart. 

 It is therefore the checking or inhibitory nerve of the heart. 



1. Course of the Fibres. These fibres leave the central 

 nervous system by the spinal accessory, and pass to the 

 heart to form connections with the cells of the cardiac 

 plexuses. 



2. Centre. The fibres arise from a centre in the medulla 

 oblongata, which can be stimulated to increased activity 

 either directly or reflexly. (1) Direct stimulation is brought 

 about by (a) sudden anaemia of the brain, as when the arteries 



