Chap, xiii.] CIRCULATION. 201 



lated products. To be complete we must besides mention the 

 numerous small capillary networks serving tbe 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 appai-atus. Our duty is to keep to 

 generalities, while merely bon-owing from particular facts those 

 of them which tire 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. 

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

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

 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 Trad^scantia 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 difierentiated, 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 movement, the canals became more 

 or less- contractile. Then by a new specialisation the contractile 



