150 THE CIRCULATION. 



Even if we estimate that length at the tenth of an inch, and 

 suppose the velocity of the blood therein to be only one inch 

 per minute, then each portion of blood may traverse its own 

 distance of the capillary system in about six seconds. There 

 would thus be plenty of time for the blood to travel through 

 its circuit in the larger vessels, in which the greatest length of 

 tube that it can have to traverse in the human subject does 

 not exceed ten feet. 



All the estimates here given are averages ; but of course the 

 time in which a given portion of blood passes from one side of 

 the heart to the other, varies much according to the organ it 

 has to traverse. The blood which circulates from the left ven- 

 tricle, through the coronary vessels, to the right side of the 

 heart, requires a far shorter time for the completion of its 

 course than the blood which flows from the left side of the 

 heart to the feet, and back again to the right side of the heart ; 

 for the circulation from the left to the right cavities of the 

 heart may be represented as forming a number of arches, vary- 

 ing in size, and requiring proportionately various times for the 

 blood to traverse them; the smallest of these arches being 

 formed by the circulation through the coronary vessels of the 

 heart itself. The course of the blood from the right side of 

 the heart, through the lungs to the left, is shorter than most 

 of the arches described by the systemic circulation, and in it 

 the blood flows, cceteris paribus, much quicker than in most of 

 the vessels which belong to the aortic circulation. For although 

 the quantity of blood contained, at any instant, in the greater 

 circulation of the body, is far greater than the quantity within 

 the lesser circulation ; yet, in any given space of time, as much 

 blood must pass through the lungs as passes in the same 

 time through the systemic circulation. If the systemic vessels 

 contain five times as much blood as the pulmonary, the blood 

 in them must move five times as slow as in these; else, the 

 right side of the heart would be either overfilled or not filled 

 enough. 



Peculiarities of the Circulation in different Parts. 



The most remarkable peculiarities attending the circulation 

 of blood through different organs are observed in the cases of 

 the lungs, the liver, the brain, and the erectile organs. The 

 pulmonary and portal circulations have been already alluded 

 to (pp. 89, 90), and will be again noticed when considering 

 the functions of the lungs and liver. 



The chief circumstances requiring notice, in relation to the 

 cerebral circulation, are observed in the arrangement and dis- 



