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BLOOD PRESSURES IN MAN, NORMAL AND PATHOLOGICAL 321 



resistance. For when the latter is excessive, e.g., during compression 

 of the aorta at the level of the diaphragm, the right heart (as well as 

 the left) becomes largely distended and the venous pressure very high. 

 But slowing of the cardiac rhythm, due to the arterial pressure, persists 

 in spite of the elevated venous pressure; the arterial pressure dominates 

 the situation, so far as the heart rate is concerned. 



As regards the direct relation of blood pressure to the normal func- 

 tioning of the vasomotor centre, Anrep and Starling have obtained 

 important evidence by a method of cross-circulation. They caused 

 the head of an animal to receive its whole blood supply from a heart- 

 lung preparation while the body of the animal retained its normal 

 blood supply from its own heart; this enabled them to study the direct 

 effects of changes of blood pressure in the head on the medullary cen- 

 tres. They found that a rise of blood pressure in the head actively 

 and almost immediately (after a latency measured in fractions of a 

 second) depresses the activity of the vasomotor centre, causing a fall 

 of blood pressure in the body generally. Changes of pressure in the 

 head induce reverse changes in the body; these are not transitory but 

 last for a long time, generally till the pressure in the brain again changes. 

 Such reversed changes in head and body, first observed by Francois- 

 Franck (49), have been studied by Hedon (GO), Tournade, Chabrol 

 and Marchand (127), Foa (47) and others. They have usually been 

 attributed to changes in the heart action through the vagus centre, 

 but such a mode of action is excluded in Anrep and Starling's experi- 

 ments. It is obvious that a mechanism of this sort must militate 

 strongly against the maintenance of an excessive pressure in the intact 

 circulation. 



In view of many facts it is clear that in persistent high pressure in 

 man the condition is not simply one of increased vascular constriction, 

 whether determined by undue activity of the vasomotor centre or by 

 chemical agents acting directly on the walls of the vessels. Simple 

 vascular constriction, raising the general pressure and the pressure in 

 the head would bring into operation various normal regulating mecha- 

 nisms such as — 1, increased control of the heart through direct action 

 of the pressure on the vagus centre together with 2, a direct synergetic 

 inhibiting influence on the vasomotor centre; 3, a reflex depressing 

 influence on the vasomotor centre through (vagus) depressor fibres 

 arising in the aorta and heart, and possibly 4, an alteration of a pressor 

 reflex influence ascending from the terminations of the vagi — a reflex 

 advocated long ago by Pavlov and recently by McDowall. 



