CHAPTER VI. 



THE CIRCULATION. 



IN animals above the very lowest grades, as also in plants, there 

 exists a particular liquid (nutritive fluid, blood, sap), which is agi- 

 tated into a circular or simply oscillating movement. By reason 

 of this movement it is permitted to reconstitute itself unceasingly, 

 to distribute the materials of nutrition to the different parts of the 

 organism, and at the same time carry away some effete products. 



In the lowest orders of animal life, as the amoebae and infusoria, 

 where no special organs are manifest and no part therefore has 

 needs differing from any other, there is found no circulatory sys- 

 tem no heart or propelling body or any blood-vessels. Its life 

 depends upon diffusion throughout its parenchyma of substances 

 brought from without and of those which must be excreted. It is 

 only as special organs show themselves and the liquids take deter- 

 mined directions toward one or another of them, that blood-vessels 

 are seen to commence; these at the same time become the recep- 

 tacles of products absorbed for the purposes of nutrition and the 

 distributors of these same materials to the various tissues of the 

 organism. 



It is, therefore, from complex organisms that the idea of a 

 perfect circulation is gained, with its admirable mechanism for 

 incessant movement whereby the fluid necessary for its growth, 

 functions, and individual life is forced to every part. Viewed as 

 a whole, the vascular system of the higher animals forms a system 

 of branching vessels or canals, closed in all parts, and not showing 

 at any point in their course the least perceptible orifice of com- 

 munication with 'the external world. Consequently, the fluids 

 which have to penetrate into the closed channels of circulation, as 

 well as those which have to emerge from them for the needs of 

 secretion and nutrition, only do so by passage through the vascular 

 walls; that is, through the finest filters imaginable. 



At a variable point in this tubular apparatus there exists an 

 organ of propulsion, the heart, which is seconded in its work by 

 auxiliary means and forces which aim to give a determined and 

 constant direction to the movement of the circulatory fluid. 



(219) 



