380 OF THE CIRCULATION OF BLOOD. 



of any such force ; and, when led to an affirmative conclusion, we shall 

 examine into its nature. No physiological fact is more clearly proved than the 

 existence, in the lower classes of Animals, as well as in Plants, of some power 

 independent of a vis a tergo, by which the circulating fluid is caused to move 

 through their vessels ( 496 498). This power seems to originate in them- 

 selves, and to be closely connected with the state of the Nutritive and Secreting 

 processes : since any thing which stimulates these to increased energy, accele- 

 rates the circulation ; whilst any check to them, occasions a corresponding 

 stagnation. It may be convenient to designate this motor force, by the name 

 of capillary power ; it being clearly understood, however, that no mechanical 

 propulsion is thence implied. On ascending the Animal scale, we find the 

 power which, in the lower organisms, is diffused through the whole system, 

 gradually concentrated in a single part ; a new force, that of the Heart, being 

 brought into operation, and the Circulation placed, in a greater or less degree, 

 under its control. Still there is evidence, that the movement of blood through 

 the capillaries is not entirely due to this ; since it may continue after the ces- 

 sation of the Heart's action, may itself cease in particular organs when the 

 Heart is still acting vigorously, and is constantly being affected in amount 

 and rapidity, by causes originating in the part itself, and in no way affecting 

 the Heart. The chief proofs of these statements will now be adverted to. 



506. 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 due to the continuance of the capillary circulation. 

 Moreover, when death has taken place suddenly from some cause, (as, for 

 instance, a sudden 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 unfre- 

 quently 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 not have taken place, unless 

 the capillary circulation were still continuing. In the early embryonic condi- 

 tion 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 vesicles loosely attached together, and can hardly be sup- 

 posed to have any great contractile power. 



507. 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 condi- 

 tion ; 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 fcstus 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 pla- 



