348 OF THE CIRCULATION OF THE BLOOD. 



direction. Irregularities of this kind, however, are more frequent when 

 the Heart's action is partly interrupted ; as it usually is by the pressure to 

 which the tadpole or other animal must be subjected, in order to allow 

 microscopic observations to be made upon its circulation. Under such 

 circumstances, the varieties in the capillary circulation, induced by causes 

 purely local, become very conspicuous; for when the whole current is nearly 

 stagnated, and a fresh impulse from the heart renews it, the movement is 

 not by any means uniform (as it might have been expected to be) through 

 the whole plexus supplied by one arterial trunk, but is much greater in 

 some of the tubes than it is in others ; the variation being in no degree 

 connected with their size, and being very different at short intervals. 



265. The movement of the blood in the capillaries of cold-blooded ani- 

 mals, after complete excision of the Heart, has been repeatedly witnessed. 

 In warm-blooded animals this cannot be satisfactorily established by experi- 

 ment, since the shock occasioned by so severe an operation much sooner 

 destroys the general vitality of the system ; but it may be proved in other 

 ways to take place.. After most kinds of natural death, the arterial system 

 is found, subsequently to the lapse of a few hours, almost or completely 

 emptied of blood ; this is partly, no doubt, the effect of the tonic contraction 

 of the tubes themselves ; but the emptying is commonly more complete than 

 could be thus accounted for, and must therefore be partly due to the con- 

 tinuance of the capillary circulation. It has been observed by Dr. Benuet 

 Dowler, 1 that in the bodies of individuals who have died from yellow fever, 

 the external veins frequently become so distended with blood uithin a few 

 minutes after the cessation of the heart's action, that, when they are opened, 

 the blood flows in a good stream, being sometimes projected to the distance 

 of a foot or more, especially when pressure is applied above the puncture, 

 as in ordinary bloodletting. It is not conceivable that the slowly-acting 

 tonicity of the arteries should have produced such a result as this; which 

 can scarcely, therefore, be attributed to anything else than the sustenance 

 of the capillary circulation by forces generated within itself. Further, it 

 has been well ascertained that a real process of secretion not unfrequently 

 continues after general or somatic death ; urine has been poured out by the 

 ureters, sweat exuded from the skin, and other peculiar secretions formed 

 by their glands ; and these changes could scarcely have taken place, unless 

 the capillary circulation were still continuing. In the early embryonic 

 condition of the highest animals, the movement of blood seems to be un- 

 questionably due to some diffused power, independent of any central impul- 

 sion ; for it may be seen to commence in the Vascular Area, before it is 

 subjected to the influence of the Heart. The first movement is Inward*, 

 instead of from, the centre; and even for some time after the circulation has 

 been fairly established, the walls of the Heart consist merely of cells loosely 

 attached together, and can hardly be supposed to have any great contractile 

 power. 



'J(!G. The last of these facts may be said not to have any direct bearing 

 on the question, whether the " capillary power" has any existence in the 

 adult condition? but the phenomena occasionally presented by the foetus, 

 at a later stage, appear decisive. Cases are of no very un frequent occur- 

 P-IICO, in which the heart is absent during the whole of embryonic life, and 

 yet the greater part of the organs are well developed. In most or all of 

 the.-e cases, it is true, a perfect twin foetus exists, of which the placenta is in 

 some degree united with that of the imperfect one; and it has been customary 



1 Researches, Critical and Experiment!!!, on the Capillary Circulation, reprinted 

 from the .New Orleans Medical and Surgical Journal, Jan. 1849. 



