THE VENOUS CIRCULATION. 677 



branches, as a rule, slightly exceed the sectional area of their common 

 trunk. In the smallest arteries this is doubtless much more marked ; 

 and on arriving at the capillaries, the increase in the total sectional 

 area of the bloodvessels, or, as it is otherwise expressed, in the capacity 

 of the capillary system, is sudden and immense. A fluid moving from 

 a small into a wider tube or channel, has its motion retarded accord- 

 ingly ; and the change of capacity, in passing from the arterial to the 

 capillary system, has been compared to that which would take place 

 in a very short cone. The relative areas of the two systems of vessels, 

 are usually, indeed, estimated, as bearing an inverse ratio to the meas- 

 ured velocity of the blood in them. Hence, according to the preceding 

 data, the sectional area of all the capillaries in the human body, would 

 be at least 300 times greater than that of the chief arterial trunks. 

 It has also been calculated to be about 400 (Volkmann), 500 (Bonders), 

 and even more than 800 times (Vierordt), greater than the area of the 

 aorta. 



Motion of the Blood through the Veins. 



The position and structure of the veins, and of their valves, have 

 been described in pp. 25 and 54. Their walls, though thinner, and 

 more easily compressible, than those of the arteries, and less elastic 

 and contractile, are very strong, the vena cava having been found to 

 require a greater force to burst it than the aorta. Collecting the blood 

 from the capillaries by minute venous radicles, the systemic veins con- 

 vey the dark blood, from all parts of the body, back to the right auricle 

 of the heart. In the limbs, the superficial veins lying beneath the 

 skin are not subjected to the pressure of muscles ; whereas the muscles 

 must press upon the sides of the deep veins. The pulmonary veins, 

 which convey bright blood from the lungs to the left auricle, are pecu- 

 liarly circumstanced, being, like the pulmonary arteries, situated en- 

 tirely within the chest. 



The blood in the veins, as indicated by opening a vein in the living 

 body, moves by an even flow, destitute of any pulsatory or jerking 

 movement; for the rhythmic character of the heart's action is already 

 lost in the capillaries, and the equable flow of blood in them, necessi- 

 tates a corresponding equability in the motion of the venous blood. 

 But the primary force which urges on the blood in the veins, is still 

 the heart's action, modified by the resilience of the arteries, which, 

 after having nearly exhausted itself, in propelling the blood through 

 the capillaries, is still adequate to move on the blood in the veins. 



The chief resistance in the circulation of the blood, takes place in 

 the capillaries, where, doubtless, it is very great ; indeed, the force of 

 the blood in the veins, as measured by hsemadynamometers fitted into 

 those vessels, varies from y^th to ^th of that of the blood in the cor- 

 responding arteries. (Poiseuille.) In the dog, the blood pressure in the 

 jugular vein, is from y^-th to -j^th of that in the carotid artery (Valen- 

 tin); but the blood pressure diminishes, in proceeding from .the branches 

 to the larger veins, and in the great veins close to the heart, the pres- 

 sure is scarcely appreciable. But certain facts seem, nevertheless, to 



