CHAP, iv.] THE VASCULAR MECHANISM. 339 



tone, is midway between extreme constriction and dilation. The 

 flow through A is determined by the resistance in A and in the 

 vascular tract which it supplies, in relation to the mean arterial 

 pressure, which again is dependent on the way in which the heart 

 is beating and on the peripheral resistance of all the small arteries 

 and capillaries, A included. If, while the heart and the rest of 

 the arteries remain unchanged, A be constricted, the peripheral 

 resistance in A will increase, and this increase of resistance will 

 lead to an increase of the general arterial pressure. Since, as we 

 have seen, 119, it is arterial pressure which is the immediate 

 cause of the flow from the arteries to the veins, this increase of 

 arterial pressure will tend to drive more blood from the arteries 

 into the veins. The constriction of A however, by increasing the 

 resistance, opposes any increase of the flow through A itself, in fact 

 will make the flow through A less than before. The whole increase 

 of discharge from the arterial into the venous system will take 

 place through the arteries in which the resistance remains un- 

 changed, that is, through channels other than A. Thus, as the 

 result of the constriction of any artery there occur, (1) diminished 

 flow through the artery itself, (2) increased general arterial 

 pressure, leading to (3) increased flow through the other arteries. 

 If, on the other hand, A be dilated, while the heart and other 

 arteries remain unchanged, the peripheral resistance in A is 

 diminished. This leads to a lowering of the general arterial 

 pressure, which in turn tends to drive less blood from the arteries 

 into the veins. The dilation of A however, by diminishing the 

 resistance, permits, even with the lowered pressure, more blood to 

 pass through A itself than before. Hence the diminished flow 

 tells all the more on the rest of the arteries in which the resistance 

 remains unchanged. Thus, as the result of the dilation of any 

 artery, there occur (1) increased flow of blood through the artery 

 itself, (2) diminished general pressure, and (3) diminished flow 

 through the other arteries. Where the artery thus constricted or 

 dilated is small, the local effect, the diminution or increase of flow 

 through itself, is much more marked than the general effects, the 

 change in blood pressure and the flow through other arteries./ 

 When, however, the area the arteries of which are affected is large, 

 the general effects are very striking. Thus if while a tracing of 

 the blood pressure is being taken by means of a manometer 

 connected with the carotid artery, the abdominal splanchnic nerves 

 be divided, a conspicuous but steady fall of pressure is observed, 

 very similar to but more marked than that which is shewn in 

 Fig. 75. The section of the abdominal splanchnic nerves causes 

 the arteries of the abdominal viscera to dilate, and these being 

 very numerous, a large amount of peripheral resistance is taken 

 away, and the blood pressure falls accordingly ; a large increase 

 of flow into the portal veins takes place, and the supply of blood 

 to the face, arms, and legs is proportionally diminished. It will 



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