492 OF THE CIRCULATION OP THE BLOOD. 



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 sup- 

 plied 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. 



522. The movement of the blood in the Capillaries of cold-blooded animals, 

 after complete excision of the Heart, has been repeatedly witnessed. In warm- 

 blooded animals, this cannot be satisfactorily established by experiment, since 

 the shock occasioned by so severe an operation much sooner destroys the gene- 

 ral 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 continuance of the capillary circulation. It 

 has been observed by Dr. Bennett Dowler, 1 that in the bodies of individuals 

 w)io have died from yellow fever (such as exhibited the remarkable post-mortem 

 movements already noticed, 328), the external veins frequently become so 

 distended with blood within a few minutes after the cessation of the heart's ac- 

 tion, that, when they are opened, the blood flows in a good stream, being some- 

 times projected to the distance of a foot or more, especially when pressure was 

 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. Fur- 

 ther, 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 unquestionably due to 

 some diffused power, independent of any central impulsion; 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 towards, instead of from, the centre ; and even 

 for some time after the circulation is 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. 



523. 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 unfrequent occurrence, 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 these 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 to attribute the circulation 

 in the latter to the influence of the heart of the former, propagated through the 

 placental vessels. This supposition has not been disproved (however improbable 

 it might seem) until recently ; when a case of this kind occurred, which was 



* "Researches, Critical and Experimental, on the Capillary Circulation," reprinted from 

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



