CHAP, iv.] THE VASCULAR MECHANISM. 311 



spinal bulb is destroyed, inhibition is not produced, however much 

 either the intestine or the mesenteric nerves be stimulated. This 

 shews that the phenomena are caused by impulses ascending 

 along the mesenteric nerves to the spinal bulb, and so affecting a 

 portion of that organ as to give rise by reflex action to impulses 

 which descend the vagus nerve or nerves as inhibitory impulses. 

 The portion of the spinal bulb thus mediating between the afferent 

 and efferent impulses may be spoken of as the cardio-inhibitory 

 centre. This centre may be thrown into activity, and so inhibition 

 produced, by afferent impulses reaching it along various nerves ; 

 by means of it reflex inhibition through one vagus may be brought 

 about by stimulation of the central end of the other. 



And we have reason to think that in a similar manner 

 augmentor impulses are developed in the central nervous system 

 either as part of a reflex chain or otherwise. 



160. So far we have been dealing with the heart of the 

 frog, but the main facts which we have stated regarding inhi- 

 bition and augmentation of the heart beat apply also to other 

 vertebrate animals including mammals, and indeed we meet 

 similar phenomena in the hearts of invertebrate animals. 



If in a mammal the heart be exposed to view by opening the 

 thorax, and the vagus nerve be stimulated in the neck, the heart 

 may be seen to stand still in diastole, with all the parts flaccid 

 and at rest. If the current employed be too weak, the result as 

 in the frog is not an actual arrest but a slowing or weakening of 

 the beats. By placing a light lever on the heart or by other 

 methods, a graphic record of the standstill, or of the slowing, of 

 the complete or incomplete inhibition may be obtained. The 

 result of stimulating the vagus is also well shewn on the blood 



FIG. 71. TRACING, SHEWING THE INFLUENCE OF CARDIAC INHIBITION ON BLOOD 

 PRESSURE. FROM A BABBIT. 



x the marks on the signal line when the current is thrown into, and y shut off 

 from the vagus. The time marker below marks seconds, the heart, as is frequently 

 the case in the rabbit, beating very rapidly. 



