CHAP, iv.] THE VASCULAR MECHANISM. 225 



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 a tube A dividing 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, besides the above 

 instance, other special and temporary variations occur due to par- 

 ticular circumstances. Thus changes of pressure may alter the 

 rapidity of flow. The cause of the flow through the whole system 

 is the pressure of the ventricular systole manifested as what we 

 have called blood-pressure. At each point along the system nearer 

 the left ventricle, and therefore further from the right auricle, the 

 pressure is greater than at a point further from the left ventricle 

 and so nearer the right auricle ; it is this difference of pressure 

 which is the real cause of the flow from the one point to the other; 

 and other things being equal the rapidity of the flow will depend 

 on the amount of the difference of pressure. Hence temporary 

 or local variations in rapidity of flow may be caused by the 

 establishment of temporary or local differences of pressure. For 

 example at any point along the arterial system the flow is in- 

 creased in rapidity during the temporary increase of pressure 

 due to the ventricular systole, i.e. the pulse, and diminished during 

 the subsequent temporary decrease, the increase and decrease 

 being the more marked the nearer the point to the heart. And 

 we shall probably meet later on with other instances. 



124. Time of the entire circuit. It is obvious from the fore- 

 going that a red corpuscle in performing the whole circuit, in 



F. 15 



