88 



CIRCULATION OF THE BLOOD. 



When the circulation has been for a long time under observation, as the animal 

 becomes enfeebled, very interesting changes in the character of the flow of blood take 

 place. The continuous stream in the smallest vessels diminishes in rapidity, and, after a 

 time, when the contractions of the heart have become infrequent and feeble, the blood 

 is nearly arrested, even in the smallest capillaries, during the intervals of the heart's 

 action, and the current becomes remittent. As the central organ becomes more and 

 more enfeebled, the circulation becomes intermittent, and the blood receives an impulse 

 from each contraction, remaining stationary during the intervals. At this time, the cor- 

 puscles cease to occupy exclusively the central portion of the vessels, and the clear layer 



FIG. 32. Portion of the lung of a 



triton, drawn under tfte microscope and magnified 150 diameters. 

 (Wagner.) 



of plasma next their walls, which was observed in the normal circulation, is no longer 

 apparent. Following this, there is an actual oscillation in the capillaries. At each con- 

 traction of the heart, the blood is forced onward a little distance, but it almost immediately 

 returns to about its former position. This phenomenon has long been observed and is 

 explained in the following way : As the heart has become enfeebled, the contractions 

 are so infrequent and ineffectual, 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 successive charges of blood sent into the arteries at each 

 ventricular systole, is lost. But, as the blood is contained in a connected system of closed 

 tubes, the feeble impulse of the heart is propagated through the vessels and produces a 

 slight impulse, 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 farther 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 



