SPECIAL PHYSIOLOGY. 



Every portion of the blood does not complete its circulation in ex- 

 actly the same time. As regards the pulmonary circulation, but little 

 difference can occur, whether a given portion of blood passes through 

 the right or the left lung, or through this or that portion of either 

 lung, on its way from the right to the left cavities of the heart ; but 

 even here, certain differences in the length of the route pursued by 

 different portions of blood, must exist. In the systemic circulation, 

 however, the differences are much more marked ; the shortest route 

 through which a portion of the blood has to pass from the left to the 

 right cavities of the heart, is that through the nutrient vessels of the 

 heart itself, and the longest route, that through the vessels of the lower 

 limbs. Different portions of the blood have, indeed, to circulate 

 through arches of varying length, and hence the time which they take 

 to traverse different parts of the body must be somewhat different. 



The slow rate of motion of the blood through the capillaries, only 

 2 inches per minute in warm-blooded animals, appears, at first sight, 

 to be opposed to the above-mentioned conclusion as regards the high 

 rate of velocity of a complete circulation ; for in that circulation, the 

 blood passes through two sets of capillary vessels, pulmonary and sys- 

 temic, besides traversing the arteries and veins of these two circula- 

 tions, as well as both sides of the heart. But it has been estimated 

 that the systemic capillaries of any given organ or tissue of the body, 

 numberless as they are, cannot measure, from the finest arteries to the 

 finest veins, more than about y^th of an inch in length ; the pulmo- 

 nary capillaries must be still shorter. According to Vierordt, the sys- 

 temic capillaries do not measure, on an average, more than /^th of an 

 inch in length, although these limits are not well defined. The blood 

 may, therefore, pass through both the systemic and pulmonary cap- 

 illaries in a period of about 3J seconds only (viz., in 3 seconds 

 through the former, and in J a second through the latter named ves- 

 sels). Assuming with Vierordt, the total period of a circulation at 

 about 28 seconds, in a man of average height, this would leave a bal- 

 ance of 24J seconds for the passage of the blood through the arterial 

 and venous channels and the heart. Supposing the mean length of 

 the arteries and veins to be, in a man of average stature, 30 inches, 

 and the average velocity of the blood in the arteries to be 6 inches 

 per second (the extremes being 10 and 2J inches), the time required 

 for the passage of , the blood through the arteries, would be 5 seconds, 

 and through the corresponding veins, the velocity in which is estimated 

 as being J to Jd that of the arteries, the time would be 12J seconds, 

 making in all 17J seconds for the circulation through the systemic 

 arteries and veins. To this must be added a period of 3J seconds, 

 for the passage of the blood through the pulmonary arteries and veins 

 (the rate of motion in them being said to be five times more rapid than 

 in the systemic arteries and veins), making a total of 21 seconds. 

 This, with the 3J seconds above mentioned as required for the pulmo- 

 nary and systemic capillary circulations, equals 24J seconds, or rather 

 more than the 23.1 seconds allowed for the complete circulation. These 

 numbers, though only approximating to the truth, still show that the 

 slow rate of the blood in the capillary portion of the circulation, or in 







