CHAP, iv.] THE VASCULAR MECHANISM. 201 



Let us suppose that the artery A is in a condition of normal 

 tone, is midway between extreme constriction and dilation. The 

 How 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. This increase 

 of pressure will tend to cause the blood in the body at large to 

 flow more rapidly from the arteries into the veins. The con- 

 striction of A however will prevent any increase of the flow 

 through it, in fact will make the flow through it less than before. 

 Hence the whole increase of discharge from the arterial into the 

 venous system must take place 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 causes the blood to flow less rapidly from 

 the arteries into the veins. The dilation of A however permits, 

 even with the lowered pressure, more blood to pass through it 

 than before. Hence the diminished flow tells all the more on the 

 rest of the arteries. 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 splanchnic nerves be divided, 

 a conspicuous but steady fall of pressure is observed, very similar 

 to that which is seen in Fig. 46. The section of the splanchnic 

 nerves causes the mesenteric and other abdominal arteries 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 be observed that the dilation of the arteries 

 is not instantaneous but somewhat gradual, the pressure sinking 

 not abruptly but with a gentle curve. 



Arterial tone then, both general and local, is a powerful 



