CHAP, xiii.] CIRCULATION. 201 



lated products. To be complete we must besides mention the 

 numerous small capillary networks serving the secretions not 

 glycogenical. 



Assuredly in this summary, so rapid and so incomplete, we 



cannot have the pretension to expound in detail the physiology 



of the diverse organs and apparatus. Our duty is to keep to 



generalities, while merely borrowing from particular facts those 



of them which are indispensable to our exposition. Nevertheless, 



it will be opportune to describe briefly the mechanism of the 



circulation in the superior vertebrates. We shall thus have 



to speak of the heart, of the arteries, of the capillary vessels, 



and finally of the veins. But previously it will not be without 



utility to present in a few lines the idea of ensemble which 



springs from our rapid summary of comparative anatomy. The 



movement of the fluids is evidently a primordial law of nutrition. 



I It is effected alike in the monocellular being and in man. First 



I of all it is by direct absorption through the exterior wall of the 



i cell. It has been observed that an active and visible exchange, 



\ when the protoplasm conveys granulations, is carried on between 



the nucleus and the protoplasm (hairs of the Tradescantid Virginia). 



, Here the circulation rigorously depends on the nutritive exchange, 



! on the summons to action of the assimilable materials, on the 



'rejection of the waste materials. In the polycellular organism, 



; little or not at all differentiated, each cell acts nearly as if it 



(were alone, and we have seen that, in plants, there is a rotatory 



movement of the protoplasm in each cell. In the inferior 



animal, in which already the nutritive labour begins to break 



'into parts, there exists a gastro-vascular apparatus. A substance 



more or less assimilable passes into a system of fine canals in 



direct communication with the digestive pouch. Then these 



.canals separate from the digestive apparatus. As in a system so 



complex, the nutritive appeal made by the anatomical elements 



and the capillarity would no longer suffice to impress on the 



; nutritive liquid the requisite mtovement, the canals became more 



or less contractile. Then by a new specialisation the contractile 



