THE CIRCULATION 243 



C. Sympathetic Fibres. The outgoing fibres are the 

 augmentors and accelerators of the heart's action. When 

 they are cut, no effect is produced, therefore the centre 

 is not constantly in action; but when the peripheral 

 end is stimulated, the rate and force of the heart are 

 increased. 



1. Course of the Fibres. These are small medullated 

 fibres. They leave the spinal cord by the anterior roots of the 

 2nd, 3rd, and 4th dorsal nerves passing to the stellate gang- 

 lion where they have their cell stations (Fig. 118). From the 

 cells in this ganglion non-medullated fibres run on in the 

 annulus of Vieussens, and from this and from the inferior 

 cervical ganglion they pass out to the muscular fibres of 

 the heart. 



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

 by stimulating various ingoing nerves, such as the sciatic ; or 

 it may be set in action from the higher nerve centres in 

 various emotional conditions. 



3. Mode of Action. These fibres seern to act directly 

 upon the muscular fibres, increasing their excitability and 

 conductivity. 



4. Result of Action 



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

 the pressure of >lood in the arteries is raised. 



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

 ventricles is increased. 



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

 increased. 



It is probable that the cardiac sympathetic also carries 

 ingoing fibres which enter the cord in the lower cervical 

 region. The pain experienced in the arm in heart disease 

 is generally thought to be due to the implication of these 

 fibres leading to the sensation which is referred to the cor- 

 responding somatic nerves (p. 146). 



The vagus is thus the protecting nerve of the heart, 

 reducing its work and diminishing the pressure in the 

 arteries. 



The sympathetic is the whip which forces the heart to 

 increased action in order to keep up the pressure in the 

 arteries. 



