THE CIRCULATION OF THE BLOOD. 



3 2 9 



The stream-bed in the aorta is relatively narrow, but widens gradually as 

 it approaches the capillaries, where it attains its maximum width; it again 

 narrows gradually as it passes into the veins, until in the venae cavae it be- 

 comes almost as narrow as in the aorta. As the combined sectional areas of 

 the venae cavae are greater than the sectional area of the aorta, the stream- 

 bed of the former never becomes as narrow as that of the latter. 



The gradual increase in the width of the stream-bed from the beginning 

 of the aorta to the middle of the capillary system, and the gradual decrease 

 in the width of the stream-bed from the middle of the capillary system to the 



Veins 



FIG. 152. SCHEMATIC ARRANGEMENT OF THE VASCULAR APPARATUS. 



terminations of the venae cavae, which result from the repeated branching 

 and subsequent reuniting, as well as its relative width in the arteries, capil- 

 laries, and veins, are shown graphically in Fig. 153. 



When the heart contracts and the intra-ventricular pressure rises above 

 the pressure in the aorta, the aortic valves are suddenly forced open and the 

 blood is driven into and through the arteries, capillaries, and veins to the 

 right side of the heart with a definite velocity and pressure. 



The 'velocity of the blood in the systemic vascular apparatus will gradu- 

 ally decrease in accordance with foregoing considerations from the aorta 

 to the middle of the capillary system in a ratio inversely proportional to the 

 total area of any given cross-section of the stream-bed, until in the capillaries 

 it will attain its minimal value, which is especially small because the 

 resistance to the flow of blood in the capillaries increases inversely as the 

 square of their diameters, while in the larger blood-vessels the increase is 

 inversely proportional to the simple diameter; the velocity will again in- 

 crease from the middle of the capillary system to the ends of the venae cavae 



