CIECULATION 



257 



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 maybe 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. The fibres seem to act (a) upon the 



FIG. 121. Simultaneous Tracing from Auricles and Ventricles. A., during 

 stimulation of the vagus ; ., during stimulation of the sympathetic. 

 Each downstroke marks a systole, each upstroke a diastole. (From ROY 

 and APAMI.) 



muscular fibres, increasing their excitability and conduc- 

 tivity; (b) upon the inhibitory mechanism, throwing it out 

 of action. 



4. Result of Action 



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

 the pressure of blood in the arteries is raised. 



17 



