THE <2!ROTLATMT ., 241 



\ O V 



due to a reflex 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 the 

 action in the blood pressure, it is called the depressor 

 nerve. 



B. Inferior Cardiac Branch of Yagus. Section of the 

 vagus or of this branch causes acceleration of the action 

 of the heart. The nerve is therefore constantly in action. 

 Stimulation 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. dwtse.tf the Fibres. These fibres leave the central ner- 

 vous system by the spinal accessory, and pass to the heart 

 to form synapses 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 

 to the head are clamped or occluded ; (b) increased venosity 

 of the blood, as when respiration is interfered with ; (c) the 

 concurrent action of the respiratory centre (see p. 295). (2) 

 Reflex stimulation is produced through many nerves. In 

 the rabbit stimulation of the 5th cranial nerve by the inhala- 

 tion of ammonia vapours has this action, and in all animals 

 stimulation of the abdominal nerves produces the same effect. 

 This reflex stimulation of the centre is used to determine its 

 position in the medulla. It can be induced after removal of 

 the brain above the medulla, but destruction of the, medulla 

 entirely prevents it. 



3. Mode of Action. These inhibitory fibres appear to act 

 by stimulating the local inhibitory mechanism in the heart ; 

 and when this has been poisoned by atropine, they cannot 

 act. According to the observation of Gaskell, they excite 

 in the heart anabolic changes, since the electric current of 

 injury is increased when they are stimulated, indicating that 

 the difference between the living part of the heart and the 

 injured part is increased (see p. 65). 



4. Result of Action. 



(a) The output of blood- from the heart is diminished, and 



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