VASOMOTOR ACTIONS. 221 



section of vaso-dilators, according, at all events, to most observers, does not 

 lead to analogous constriction or diminution of dilatation ; all that is observed 

 is a transient increase of dilatation due probably to the section acting as a 

 transient stimulus to the nerve at the place of section. But before we study 

 the use made by the central nervous system of vasomotor nerves, it will be 

 best to consider briefly some features of 



The Effects of Vasomotor Actions. 



157. A very little consideration will show that vasomotor action is a 

 most important factor in the circulation. In the first place the whole flow 

 of blood in the body is adapted to and governed by what we may call the 

 general tone of the arteries of the body at large. In a normal condition 

 of the body a very large number of the minute arteries of the body are in 

 a state of tonic, i. e., of moderate, contraction, and it is the narrowing due to 

 this contraction which forms a large item of that peripheral resistance which 

 we have seen to be one of the great factors of blood-pressure. The normal 

 general blood-pressure, and, therefore, the normal flow of blood, is in fact 

 dependent on the " general tone " of the minute arteries. 



In the second place local vasomotor changes in the condition of the 

 minute arteries changes i. e., of any particular vascular area have very 

 decided effects on the circulation. The changes, though local themselves, 

 may have effects which are both local and general, as the following con- 

 siderations will show : 



Let us suppose that the artery A is in a condition of normal tone, is mid- 

 way between extreme constriction and dilatation. The flow through A is 

 determined by the resistance in A, and in the vascular tract which it sup- 

 plies in relation to the mean arterial pressure, which is again 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 ( 108), 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 in- 

 creasing 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 unchanged, that is, through 

 channels other than A. Thus, as the result of the constriction of an artery 

 there occur (1) diminished flow through the artery itself, (2) increased gen- 

 eral 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 dilatation 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 dilatation 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, 



