Chap, iv.] THE VASCULAR MECHANISM. 177 



than through a. For the same reason, water flowing along a river 

 impelled by one force, viz. that of gravity, rushes rapidly through 

 a ' narrow,' and flows sluggishly when the river widens out into 

 a ' broad.' The flow through B will be similarly slackened if B, 

 instead of being simply a single enlargement of the tube A, consists 

 of a number of small tubes branching out from A, with a united 

 sectional area greater than the sectional area of A. In each of 

 such small tubes, at the line c, for instance, the flow will be slower 

 than at a, where the small tubes branch out from A, or at b, where 

 they join again to form a single tube. Hence it is that the blood 

 rushes swiftly through the arteries, flows slowly through the 

 capillaries, but quickens its pace again in the veins. 



An apparent contradiction to this principle that the rate of 

 flow is dependent on the width of the bed is seen in the case 

 where, the fluid having alternative routes, one of the routes is 

 temporarily widened. Suppose that a tube A divides into two 

 branches of equal length x and y, which unite again to form the 

 tube V. Suppose, to start with, that x and y are of equal 

 diameter: then the resistance offered by each being equal, the 

 flow will be equally rapid through the two, being just so rapid 

 that as much fluid passes in a given time through x and y together 

 as passes through A or through V. But now suppose y to be 

 widened : the widening will diminish the resistance offered by y, 

 and, in consequence, supposing that no material change takes 

 place in the pressure or force which is driving the fluid along, more 

 fluid will now pass along y in a given time than did before , that is 

 to say, the rapidity of the flow in y will be increased. It will be 

 increased at the expense of the flow through x, since it will still 

 hold good that the flow through x and y together is equal to the 

 flow through A and through V We shall have occasion later on 

 to point out that a small artery, or a set of small arteries, may 

 be more or less suddenly widened, without materially affecting the 

 general blood pressure which is driving the blood through the 

 artery or set of arteries. In such cases the flow of blood through 

 the widened artery or arteries is, for the time, being increased in 

 rapidity, not only in spite of, but actually in consequence of the 

 artery being widened. 



It must be understood, in fact, that this dependence of the 



rapidity of the flow on the width of the bed applies to the general 



rate of flow of the whole circulation ; and that while, on account of 



the width of the bed, the flow through the capillaries is slower 



than through the small arteries and veins, that through the small 



arteries slower than through the larger arteries, and that through 



the small veins slower than through the larger veins, the actual 



rapidity in any individual capillary, small artery or small vein, or 



in any individual sets of these, varies largely from time to time, 



owing to changes of circumstances, prominent among which are 



changes in the resistance to the flow, — changes which, as we shall 



12 



