CIRCULATION. 395 



by different observers. The arterial and venous systems, treated as each a 

 single tube, may be compared roughly to two funnels, each having its nar- 

 row end at the heart. The very wide and very short single tube of the capil- 

 lary system may be imagined to connect the wide ends of the two funnels. 

 Equal quantities of blood pass in equal times any two points of the collec- 

 tive blood-path between the left ventricle and the right auricle. Therefore 

 where the blood-path is wide, these quantities must move slowly, and swiftly 

 where the blood-path is narrow. It is owing, then, to the rapid widening of 

 the arterial path that the speed declines, like the pressure, toward the capilla- 

 ries. It is owing to the huge relative calibre of the path at the capillaries 

 that in them the speed is by far the least while the same volume is passing 

 that passes a point in the narrow aorta in the same time ; it is owing to the 

 steady narrowing of the venous path toward the heart that the venous blood 

 is constantly quickening its speed while its pressure is falling. As the calibre 

 of the venous system is greater than that of the arterial, the average speed in 

 the veins is probably less than in the arteries. As the collective calibre of 

 the veins which enter the right auricle is greater than that of the aorta, the 

 blood probably moves into the heart less swiftly than out of it ; though of 

 course equal quantities enter and leave it in equal times provided those times 

 are not mere fractions of a beat. In connection with this it is significant 

 that the entrance of blood into the heart takes place during the long auric- 

 ular diastole, while its exit is limited to the shorter ventricular systole. 



Time Spent by the Blood in a Systemic Capillary. The width of the 

 path, then, determines the slow movement of the blood in the areas where it is 

 fulfilling its functions ; the narrowness of the path, the swiftness of move- 

 ment of the blood in leaving and returning to the heart. We have seen (p. 

 371) that a particle of blood may make the entire round of a dog's circulation 

 in from fifteen to eighteen seconds. If we assume the systemic capillary flow 

 to be at the rate of 0.8 millimeter in one second, the blood would remain about 

 0.6 of a second in a systemic capillary half a millimeter long. Slow as is the 

 capillary flow, it thus appears that it is none too slow to give time for the uses 

 of the blood to be fulfilled. 



F. THE FLOW OP BLOOD THROUGH THE LUNGS. 

 The blood moves from the right ventricle to the left auricle under the 

 same general laws as from the left ventricle to the right auricle. Certain dif- 

 ferences, however, are apparent, and must be noted. One difference is that 

 the collective friction is less in the pulmonary than in the systemic vessels, 

 and that therefore the resistance to be overcome by each contraction of the 

 right ventricle is less than that opposed to the left ventricle. Accordingly it 

 appears from dissection that the muscular wall of the right ventricle is much 

 thinner than that of the left. No accurate measurements can be made of the 

 normal pressure and speed of the blood in the arteries, capillaries, and veins 

 of the lungs, because they can be reached only by opening the chest and 

 destroying the mechanism of respiration, and thereby disturbing the normal 



