372 THE CIRCULATION OF THE BLOOD. [CHAP. XXVIII, 



the most energetic affinity, will move with the greatest velocity, 

 and may even drive the other liquid entirely before it." 



In plants, the rise of the ascending sap from the ground, 

 results from the attractive force of the spongioles. These appro- 

 priate certain elements contained in the fluid, and exercise a more 

 energetic attraction on a new supply, which pushes the former 

 before it. Thus the sap ascends to the leaves, pushed on by succes- 

 sive new portions attracted to the spongioles. At the leaves, a new 

 force of a similar kind, but due to the action of light, draws it on, 

 and causes it to push before it the newly-formed latex or elaborated 

 sap, the flow of which is promoted by its affinity for the vegetable 

 tissues which it permeates. 



In the systemic circulation of animals, the arterial blood has a 

 great affinity for the tissues to which it is brought by the capillary 

 system. This force of attraction draws on the blood from the 

 arterial side of that system, with a power which helps to propel on 

 the de-oxygenized blood into the venous radicles. In the pulmonary 

 circulation, venous blood is conveyed to the air-cells by the pulmo- 

 nary arteries. This kind of blood has a great affinity for the oxygen 

 which is being continually brought to those cells by the movements 

 of respiration. It is, therefore, forcibly attracted to the air-cells, 

 and, being charged with oxygen, is pushed on by the succeeding 

 portions of venous blood, which the same force is constantly 

 attracting. 



It is by the influence of an attractive force, such as Professor 

 Draper describes, that we can best explain the continuance of a 

 complex circulation in many of the lower animals in which no 

 central organ of impulsion exists, as in some of the Polypifera, 

 and of the Articulata. In the sponge, the remarkable currents of 

 water which flow through the various channels that penetrate its 

 substance, are maintained without any special propelling organ 

 whatever. And the beautiful cyclosis in Chara and Valisneria 



/ 



affords a striking instance of a circulation without vis a tergo. 



In the vascular area of the egg a circulation exists before a pro- 

 pelling organ. And in the acardiac foetus a similar circulation 

 exists, although in general it has such a connexion with a second 

 perfect foetus, that the heart of the latter may influence the circula- 

 tion of the former. But that a foetus may grow to a considerable 

 size, and have its various tissues well developed without any con- 

 nexion of the twin foetus, by means exclusively of a circulation 

 of its own, of which a heart forms no portion, or upon which 

 it can exercise but a very remote influence, is shewn by the case 



