CHAP, iv.] THE VASCULAR MECHANISM. 177 



To these may be added as subsidiary modifying events : 



3. Changes in the peripheral resistance of the capillaries due 

 to alterations in the adhesiveness of the capillary walls or to 

 other influences arising out of the as yet obscure relations existing 

 between the blood within and the tissue without the thin per- 

 meable capillary walls, and depending on the vital conditions of the 

 one or of the other. Such changes causing an increase of peri- 

 pheral resistance are seen to a marked degree in the pathological 

 condition known as stasis. 



4. Changes in the quantity of blood in circulation. 



The two first and chief classes of events (and probably the 

 third) are directly under the dominion of the nervous system. It 

 is by means of the nervous system that the heart's beat and the 

 calibre of the minute arteries are brought into relation with each 

 other, and with almost every part of the body. It is by means of 

 the nervous system acting either on the heart, or on the small 

 arteries, or on both, that a change of circumstances affecting either 

 the whole or a part of the body is met by compensating or 

 regulative changes in the flow of blood. It is by means of the 

 nervous system that an organ has a more full supply of blood when 

 at work than when at rest, that the stream of blood through the 

 skin rises and ebbs with the rise and fall of the temperature of the 

 air, that the work of the heart is tempered to meet the strain of 

 overfull arteries, and that the arterial gates open and shut as the 

 force of the central pump waxes and wanes. Each of these vital 

 factors of the circulation must therefore be considered in connection 

 with those parts of the nervous system which are concerned in 

 its action. 



F. 



12 



