HEART 423 



2. The Centre is in the medulla, and it may be reflexly 

 stimulated through various ingoing nerves, such as the 

 sciatic ; or it may be set in action from the higher nerve 

 centres in various emotional conditions. It is also called 

 into play, along with inhibition of the action of the vagus, when 

 increased venous inflow in diastole leads to over-distension. 



3. Mode of Action. — The fibres seem to act (a) upon the 

 muscular fibres, by increasing their excitability and conduc- 

 tivity ; (5) upon the inhibitory mechanism, by throwing it 

 out of action. 



4. Results of Action (fig. 182, B). 



(a) The rate of the rhythmic movements of auricles and 

 ventricles is increased. 



(h) The force of contraction of auricles and ventricles is 

 increased. 



Thus, the output of blood from the heart is increased, 

 and the pressure of blood in the arteries is raised. 



It is probable that the cardiac sympathetic also carries 

 ingoing fibres which enter the cord in the lower cervical 

 region. The pain experienced down the inside of the arm 

 in heart disease in man is generally thought to be due to the 

 implication of these fibres leading to sensations which are 

 referred to the corresponding somatic nerves. 



The vagus is thus the protecting nerve of the heart, 

 reducing its work and diminishing the pressure in the 

 arteries, and it is called into action when the systolic 

 pressure rises too high, while it is inhibited when the venous 

 inflow is too much increased. 



The sympathetic is the whip which forces the heart to 

 increased action in order to keep up the pressure m the 

 arteries, and it is brought into action by increase in the 

 venous inflow, so that the intrinsic response is supplemented 

 by this extrinsic effect. 



