SEC. 8. THE MUTUAL RELATIONS AND THE CO-ORDI- 

 NATION OF THE VASCULAR FACTORS. 



The foregoing considerations shew how complicated, and 

 sensitive, and therefore how useful, is the vascular mechanism. 

 It may be worth while briefly to summarize the relations of the 

 different factors, and to point out the manner in which they are 

 made to work in harmony for the good of the body. 



Two facts stand out prominent above all others: (1) the heart's 

 beat may be made slow by vagus inhibition, and, on the other 

 hand, quickened either by withdrawal of the constant inhibitory 

 influence exercised by the cardio-inhibitory centre, or by the direct 

 action of accelerating mechanisms. (2) The peripheral resist- 

 ance may be increased or diminished, the increase and decrease 

 being due either to increased or diminished action of the vaso- 

 motor centres which preside over arterial tone, or to the action of 

 special constrictor or dilator fibres. 



These two facts are, by the mediation of the nervous system, 

 placed in mutual regulative dependence on each other. Thus, if 

 with a given peripheral resistance, and proportionate blood-pressure, 

 the heart begins to beat violently, afferent impulses passing up the 

 depressor nerves diminish peripheral resistance (by opening the 

 splanchnic flood-gates), and prevent the rise of blood-pressure 

 which would otherwise take place. In this way a delicate organ, 

 such for instance as the retina, is sheltered from the turbulence of 

 the heart by the flow of blood being diverted to the less noble 



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