RAPIDITY OF THE CAPILLARY CIRCULATION. 289 



and is explained in the following way : As the heart has 

 become enfeebled, the contractions are so infrequent and in- 

 effectual, that during their intervals the constant flow in the 

 capillaries is entirely arrested ; for the arterial pressure, which 

 is its immediate cause, and which is maintained by the suc- 

 cessive charges of blood sent into the arteries at each ventric- 

 ular systole, is lost. But as the blood is contained in a con- 

 nected system of closed tubes, the feeble impulse of the heart 

 is propagated through the vessels and produces a slight im- 

 pulse, even in the smallest capillaries, which dilates them 

 and forces the fluid a little distance. As soon, however, as 

 the heart ceases to contract, the current is arrested, and the 

 blood, meeting with a certain amount of obstruction from the 

 fluid in the small veins, which are still further removed from 

 the heart, is made to return to its former position. 



This phenomenon continues for a short time only, for the 

 heart soon loses its contractility, and the circulation in all the 

 vessels is permanently arrested. 



Rapidity of the Capillary Circulation. The circulation 

 in the capillaries of a part is subject to such great variations, 

 and the differences in different situations are so considerable, 

 that it is impossible to give any definite rate which will 

 represent the rapidity of the capillary circulation. It is for 

 this reason that it has been found impracticable to estimate 

 the capacity of the capillary, as compared with the arterial, 

 system. The rapidity of the flow of blood is by no means 

 as great as it appears in microscopic examinations; being, 

 of course, exaggerated in proportion to the magnifying power 

 employed. It is, nevertheless, to microscopic investigations 

 that we are indebted for the scanty information we possess 

 on this subject. The estimates which have been made by 

 various observers refer generally to cold-blooded animals, and 

 have been arrived at by simply calculating the time occupied 

 by a blood-corpuscle in passing over a certain distance. Hales, 

 who was the first to investigate this question, estimated that 

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