222 THE CIRCULATION OF THE BLOOD 



heart and partly in the peripheral resistance. The nerve control of the 

 heart is effected through the vagus and sympathetic nerves, and that 

 exercised on the blood vessels, through the so-called vasoconstrictor and 

 vasodilator nerves. 



The activity of the nerve centers from which the cardiac and vaso- 

 motor impulses are discharged is controlled by afferent impulses com- 

 ing from the various regions of the body. When a gland becomes more 

 active, we must suppose that stimulation of the sensory fibers has caused 

 afferent impulses to be transmitted to the cardiac and vasomotor centers, 

 upon which they act in such a way as to produce increased heart ac- 

 tion and a local dilatation of the blood vessels of the active gland, with 

 perhaps a constriction of the blood vessels of the rest of the body. 



THE NERVE CONTROL OF THE HEARTBEAT 



The Vagus Control 



With regard to the control exercised through the vagus nerve, we have 

 already seen that the cutting of the two nerves in the neck causes the 

 heart to quicken and the arterial blood pressure to rise, whereas a 

 stimulation of the peripheral end of the nerve causes the heart to be- 

 come slowed, if not stopped altogether, and the blood pressure to fall. 



For the more detailed investigation of the nature of the vagus control 

 of the heart, it is necessary to observe the exposed heart itself an ex- 

 periment which, for obvious reasons, can be most simply performed in 

 a cold-blooded animal, such as the frog or turtle, but which can also 

 be performed in mammals provided artificial respiration is maintained. 

 The general effect of the vagus in both groups of animals is the same, 

 although apparent differences may exist on account of the relative im- 

 portance of the different parts of the heart in the origination and propa- 

 gation of the heartbeat. 



The Cold-Blooded Heart. If the vagus nerve on the right side in the 

 turtle (the left nerve is usually more or less inactive in this animal) is 

 stimulated with a very feeble electric current, while simultaneous records 

 are being taken of the contractions of the auricles and ventricles in the 

 manner shown in the accompanying tracing (Fig. 64), it will often be 

 found that there is a weakening of the auricular beats without any change 

 in those of the ventricle. If the strength of stimulus is somewhat in- 

 creased, the auricular beat, besides becoming weaker, will also become 

 slower, but meanwhile the ventricular, although also slower, may become 

 distinctly stronger. At first sight this result may be a little confusing, 

 because it would seem to indicate that the vagus nerve weakens the auricu- 



