CHAP, iv.] THE VASCULAR MECHANISM. 319 



nerves of the skin, and probably in all cases, are normally in a 

 state of moderate activity (so long as they remain in connection 

 with the central nervous system), the moderate activity maintain- 

 ing that moderate constriction which we spoke of above as ' tone,' 

 the vaso-dilators appear to possess no such continued activity. 

 Section of vaso-constrictor fibres leads to loss of tone, diminution 

 of constriction, lasting, as we shall see, for some considerable time ; 

 but section of vaso-dilators, according at all events to most 

 observers, does not lead to analogous constriction, or diminution of 

 dilation ; all that is observed is a transient increase of dilation 

 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 vaso-motor nerves it will 

 be best to consider briefly some features of 



The Effects of Vaso-motor Actions. 



171. A very little consideration will shew that vaso-motor 

 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 vaso-motor changes in the condition 



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



have very decided effects on the circulation. These changes, 



though local themselves, may have effects which are both local 



"and general, as the following considerations will shew. 



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

 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 



