n THE CARDIO-INHIBITORY CENTRE 77 



this way the heart is indirectly connected with all parts 

 of the body, so that by nervous agencies its beat may be 

 made to vary. For instance when a person faints from a 

 sudden emotion, an influence is started in the brain, 

 passes down to the centre in the spinal bulb, increases 

 its action and stops for a time the beating of the heart. 

 Or again, fainting may result from a blow on the 

 stomach ; in this case the influence starts at the part 

 struck, and passing up the spinal cord to the cai'dio- 

 inhibitory centre, increases its activity and leads as 

 before to stoppage of the heart. The rapid and violent 

 beating of the heart which we speak of as "palpitation" 

 may on the other hand be often due to some emotion 

 which in this case lessens the activity of the centre and 

 hence diminishes the restraint which it ordinarily 

 exerts over the heart. But of course palpitation may 

 also at times be due to impulses reaching the heart 

 along those nerves which we have described above as the 

 accelerators. 



Our knowledge of the existence and position of the 

 cardio-inhibitory centre is quite clear and definite. It is 

 possible that a cardio-augmentor (-accelerator) centre may 

 also exist, but at present we have no exact knowledge of 

 its existence ; hence in the accompanying figure (Fig. 23) 

 the accelerator nerves are shown as originating in the 

 central nervous system, but not arising from any de- 

 finitely localised centre. 



18. Tlie Proofs of the Circulation. — The evidence 

 that the Ijlood circulates in man, although perfectly con- 

 clusive, is almost all indirect. The most important points 

 in the evidence are as follows : — 



In the first place, the disposition and structure of the 

 organs of circulation, and more especially the arrange- 

 ment of the various valves, will not, as was shown by 

 Harvey, permit the blood to flow in any other direction 



