560 OF THE CIRCULATION OF BLOOD. 



enlargement, must be due, not to simple dilatation merely, but to increased 

 nutrition ; since we find that their walls are thickened as well as extended. 

 And, on the other side, when slow contraction occurs in these tubes, as a 

 consequence of disease, it must be in part occasioned by atrophy; since 

 their nutrition is so much diminished, that in time they almost entirely disap- 

 pear, a portion of a large artery occasionally shriveling into a ligamentous 

 band. 



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

 convey the blood through the capillary system ; that, namely, which con- 

 cerns the agencies existing in the Capillaries themselves. Many discussions 

 on this subject may be found in Physiological writings ; and it has so imme- 

 diate a bearing on one of the most important questions in Pathology, the 

 nature of Inflammation, that it deserves the fullest attention. The chief 

 question in debate, is the degree in which the Capillary circulation is influ- 

 enced by any other agency than the contractile power of the Heart and Arte- 

 rial system ; some Physiologists maintaining that this alone is sufficient 

 to account for all the phenomena 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 Arte- 

 ries 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 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 u tergo, by which the circulating fluid is 

 caused to move through their vessels ( 712 716). This power seems to 

 originate in themselves, and to be closely connected with the state of the 

 Nutritive and Secreting processes : since anything which stimulates these to 

 increased energy, accelerates the circulation : whilst any check to them occa- 

 sions 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 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. 



734. 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 ihesize 

 of the Capillary tubes, have been already referred to ( 219). Others, how- 

 ever, are manifested in great and sudden alterations in the velocity of the cur- 

 rent ; 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 reversion, 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 cur- 



