CIRCULATION. 75 



been seen to continue after death, which could not take place unless 

 the capillary circulation were still continued. In the embryo, the 

 blood circulates before the heart is formed ; and instances are not 

 wanting in which the heart is entirely absent during embryonic life, 

 and yet the greater part of the organs were well developed. In the 

 latter case the circulation must be due to capillary power. The 

 local determination that takes place whenever the processes of nutri- 

 tion and secretion are carried on, and the increased rapidity of the 

 movement without corresponding increase of the heart's action, also 

 go to support the idea of an independent capillary power. 



These facts, and others to prove that the circulation in a part may 

 be arrested, while the heart is still acting, and the vessels pervious, 

 seem to be favourable to the support of the belief in the existence of 

 such a power. 



In regard to the nature of the power, it cannot be mechanical^ since 

 no movement of contraction or dilatation has ever been seen. The 

 conditions under which the power in question uniformly operates, may 

 be thus expressed : — " Whilst the injection of blood into the capillary 

 vessels of every part of the system is due to the action of the heart, 

 its rate of progress through those vessels is greatly modified by the 

 degree of activity in the processes to which it should be normally sub- 

 servient in them ; — the current being rendered more rapid by an in- 

 crease in their activity, and being stagnated by their depression or 

 total cessation. 



" Thus the capillaries seem to have a distributive power over the 

 blood, regulating the local circulation independently of the heart's 

 action, and in obedience to the necessities of each part."* 



The views of Prof. Draper in relation to the dependence of the 

 circulation of the sap in plants upon chemical changes in the cir- 

 culating fluid, assist to explain very satisfactorily the capillary cir- 

 culation in animals. A brief and modified summary of these views 

 is here given. Experiments on inorganic bodies show, that if two 

 liquids communicate with each other through a capillary tube, for 

 the walls of which they have an unequal affinity, the liquid which 

 has the greater affinity will be absorbed most energetically, and drive 

 the other before it. This is what seems to take place in the organ- 

 ized tissues, permeated by nutritious fluid. The particles of this 

 fluid, and the solid matter through which it is distributed, have a 

 certain affinity for each other, which is exercised in the nutritive 

 changes, to which the fluid becomes subservient during the course of 

 its circulation. Certain matters are drawn from it in one part to 

 carry on the nutritive process ; in another, to accomplish the func- 

 tion of secretion. The fluid, which has given up to one tissue some 

 of its materials, has no longer the same affinity for that tissue ; it is 



* Carpenter's Princip. p. 564. 



