CHAP, iv.] THE VASCULAR MECHANISM. 223 



but is concerned with the vital condition of the tissues. When the 

 tissue is in health, a certain resistance is offered to the passage of 

 blood through the capillaries, and the whole vascular mechanism is 

 adapted to overcome this resistance to such an extent that a 

 normal circulation can take place. When the tissue becomes 

 affected, the disturbance of the equilibrium between the tissue 

 and the blood may as in inflammation so modify the flow as to lead 

 to the abnormal escape from the blood of various constituents, or as 

 in stasis so augment the resistance that the passage of the blood 

 becomes difficult or impossible. And it is quite open to us to 

 suppose that there are conditions the reverse of stasis, in which the 

 resistance may be lowered below the normal, and the circulation 

 in the area quickened. 



Thus the vital condition of the tissue becomes a factor in the 

 maintenance of the circulation ; and it is possible, though not yet 

 proved, that these vital conditions are directly under the dominion 

 of the nervous system. 



It is perhaps hardly necessary to observe that the considerations 

 urged above are quite distinct from what is sometimes spoken of under 

 the name of 'capillary' force, as an agent of the circulation. If by 

 capillary force it is intended to refer to the rise of fluids in capillary 

 tubes, it is evident that since such phenomena are the results of 

 adhesion, capillarity can only be a greater or less hindrance to the flow 

 of blood, seeing that this is propelled by a force (the heart's beat) 

 which has been proved by experiment to be equal to the task of 

 driving the blood from ventricle to auricle through the capillary regions. 

 If by capillary force it is meant that the tissues have some vital power 

 of withdrawing the fluid parts of the blood from the small arteries and 

 thus of assisting an onward flow, it becomes necessary also to assume 

 that they have as well the power of returning the fluid parts to the 

 veins. Both these assumptions are unnecessary and without foundation. 



