420 VETERINARY PHYSIOLOGY 



inferior cardiac branch ; and the fall of blood pressure, 

 which is the most manifest effect, is due to a reflex dilata- 

 tion 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. 



2nd. 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. Stimu- 

 lation of its central end has no effect ; stimidation of its 

 peripheral end causes a slowing or stoppage of the heart. 

 Less blood is pumped into the arteries, and the pressure in 

 them falls (fig. 188). It is therefore the checking or in- 

 hibitory nerve of the heart. The right vagus is chiefly 

 connected with the sino-auricular node, and its stimulation 

 slows the rate of the heart. The left vagus is specially 

 connected with the auriculo- ventricular node, and its stimu- 

 lation tends to slow or prevent conduction of contraction 

 from the auricles to the ventricles. But these two actions 

 are not always clearly differentiated. 



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 cells 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 to the head are 

 clamped or occluded ; (6) increased venosity of the blood., 

 as when respiration is interfered with ; (c) the concurrent 

 action of the respiratory centre (see p. 585). 



(2) Reflex stimulation is produced through many nerves, 

 e.g. those of the abdomen — a point of great importance in 

 abdominal surgery. The superior cardiac branch of the vagus 

 from the ventricles and wall of the aorta is stimulated Avhen the 

 arterial pressures rises and leads to a reflex slowing of the 

 heart, which, along with the dilatation of the abdominal 

 vessels, reduces the pressure. 



