THE VASCULAR MECHANISM. 



221 



it seems to act mainly by lessening the conductivity of the bundle of 

 flif, and the auricles continue to beat at their usual rate, whereas the 

 ventricles beat infrequently or not at all. 



The Sympathetic Fibres. Stimulation of the sympathetic nerves, 

 either in the frog or in the mammal, quickens the heart, and for this 

 reason they are called accelerator nerves ; usually the force of the heart 



FIG. 83. Stimulation of peripheral end of one vagus at X. 



Note the inhibition of the heart and fall of blood pressure. B.P., arterial pressure recorded 

 by mercurial manometer. 



is also increased. The effect is only produced after a latent period of 

 some seconds, and lasts for a little time after the cessation of the 

 stimulation (fig. 84). In the mammal the blood pressure may rise 

 slightly or may be unaffected. 



Cardiac Reflexes. The efferent fibres of the vagus arise from a 

 collection of nerve cells lying in the medulla oblongata, and known as 

 the vagus centre. Impulses are constantly passing from the centre 

 down the vagus ; these exert a restraining force on the rate of the 

 heart beat and tend to inhibit it. This action of the vagus centre is 



