MOVEMENT OF THE BLOOD .IN THE CAPILLARIES. 347 



which supply them. 1 Donders, on similar data, estimates it at 500 times 

 greater, and Vierordt at from 800 to 850 times. 2 



263. That the movement of the blood through the Capillary system of 

 vessels, is mainly dependent upon the force which it derives from the Heart 

 and from the coats of the Arteries, is a matter altogether beyond dispute. 

 But it is a most important question, not merely in itself, but in its bearing 

 on one of the fundamental questions of Pathology, the nature of inflam- 

 mation, whether the Capillary circulation is influenced by any other agency 

 than the contractile power of the Heart and Arterial system ; some Phys- 

 iologists maintaining that this alone is sufficient to account for all the phe- 

 nomena of the Capillary circulation ; and others asserting that it is neces- 

 sary to admit some supplementary force, which may be exerted either to 

 assist, retard, or regulate the flow of blood from the Arteries into the Veins. 

 We shall first consider the evidence which may justify an affirmative con- 

 clusion as to the existence of such force; and shall then examine into its 

 nature. No physiological fact seems to the Author to be 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 nutritive 

 fluid is caused to move through their vessels. 3 This power appears to orig- 

 inate in the circulation itself, and to be closely connected with the state of 

 the Nutritive and Secreting processes: since anything which stimulates 

 these to increased energy, accelerates the movement ; whilst any check to 

 them occasions a corresponding stagnation. It may be convenient to des- 

 ignate 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 organ- 

 isms, 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 cessation 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. 



264. When the flow of blood through the capillaries of a transparent 

 part, such as the web of a Frog's foot, is observed with the microscope, it 

 appears at first to take place with great evenness and regularity. But on 

 watching the movement for some time, various changes may be observed, 

 which cannot be attributed to the heart's influence, and which show that a 

 certain regulating or distributive power exists in the walls of the capillaries, 

 or in the tissues which they traverse. Some of these changes, involving 

 variations in the size of the capillary tubes, have been already referred to 

 ( 260). Others, however, are manifested in great and sudden alterations 

 in the velocity of the current ; which cause a marked difference in the rates 

 of the movement of the blood through the several parts of the area under 

 observation. Sometimes this variation extends even to the entire reversal, 

 for a time, of the direction of the movement, in certain of the transverse 

 or communicating branches; the flow always taking place, of course, from 

 the stronger towards the weaker current. Not unfrequently, an entire stag- 

 nation of the current in some particular tube precedes this reversal of its 



1 Hamodynamik, pp. 184, 



2 Die Erscheinun^cn urid Gesetzc cler Stromgeschwindigkeit des Blutes, 1858. 



3 See Principles of Comparative Physiology, chap. v. 



