MOTION OF THE BLOOD IN THE CAPILLARIES. 491 



4. Motion of the filood in the Capillaries. 



520. We now come to the last head of the inquiry into the powers which 

 convey the blood through the capillary system; that, namely, which concerns 

 the agencies existing in the Capillaries themselves. Many discussions on this 

 subject may, be found in Physiological writings: and it has so immediate a 

 bearing on one of the most important questions in Pathology the nature of In- 

 flammation that it deserves the fullest attention. The chief question in debate 

 is the degree in which the Capillary circulation is influenced by any other 

 agency than the contractile power of the Heart and Arterial system; some 

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

 nomena of the Capillary circulation; and others asserting, that it is necessary 

 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 what evidence there is of the existence of any such force; and, when 

 led to an affirmative conclusion, we shall examine into its nature. No physio- 

 logical 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. 1 This power seems to originate 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 con- 

 venient 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 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 state- 

 ments will now be adverted to. 



521. 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 ( 292). 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 ex- 

 tends 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 unfre- 

 quently, an entire stagnation of the current in some particular tube, precedes 

 this reversal of its direction. Irregularities of this kind, however, are more 

 frequent when the Heart's action is partly interrupted; as it usually is by the 



1 See "Princ. of Phys., Gen. and Comp.," CHAP, xn., Am. Ed. 



