MOTION OF THE BLOOD IN THE ARTERIES. 561 



rent. Not unfrequently an entire stagnation of the current, in some particular 

 tube, precedes this reversal of its direction. Irregularities of this kind, how- 

 ever, are more frequent when the Heart's action is partially 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 cir- 

 culation. Under such circumstances, the varieties in the capillary circulation, 

 induced by causes purely local, become very conspicuous ; for when the 

 whole current has nearly stagnated, and a fresh impulse from the heart re- 

 news 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 in- 

 tervals. 



735. 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 de- 

 stroys 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 continuance of 

 the capillary circulation. Moreover, when death has taken place suddenly 

 from some cause (as, for instance, a violent electric shock), that destroys the 

 vitality of the whole system at once, the arterial tubes are found to contain 

 their due proportion of blood. 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 excluded from the 

 skin, and other peculiar secretions formed by their glands; and these changes 

 could not have taken place unless the capillary circulation were still continu- 

 ing. 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 the development 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 at- 

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

 power. 



736. 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 con- 

 dition ; 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, however, 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 circula- 

 tion 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 submitted to the most careful examination by an 

 accomplished anatomist ;* and this decisive result was obtained, that it seemed 



* See Dr. Houston in the Dublin Medical Journal, 1837. An attempt has been recently 

 made by Dr. M. Hall (Edinb. Monthly Journal, 1843) to disprove Dr. Houston's inferences; 



