206 THE VITAL FUNCTIONS. 



as in other departments of the economy of nature, 

 changes are scarcely ever abruptly made, but gene- 

 rally by slow and successive steps. In the very 

 lowest tribes of Zoophytes, the modes by which 

 nutrition is accomplished hardly differ from those 

 adopted in the vegetable kingdom ; where, as we 

 have already seen, the nutritive fluids, instead of 

 being confined in vessels, appear to permeate the 

 cellular tissue, and thns immediately supply the 

 solids with the materials they require ; for, in these 

 animals, the nourishment which has been prepared 

 by the digestive cavities is imbibed by the solids, 

 after having transuded through the sides of these 

 organs, and without its being previously collected 

 into other, and more general cavities. This mode 

 of nutrition, suited only to the torpid and half ve- 

 getative nature of zoophytes, has been denominated 

 noiirishment hy imbibition, in contradistinction to 

 that by circulation; a term, which, as we have 

 seen, implies, not merely a system of canals, such 

 as those existing in Medusae, where there is no 

 evidence of the fluids really circulating, but an 

 arrangement of ramified vessels, composed of mem- 

 branous coats, through which the nutrient fluid 

 moves in a continued circuit. 



The functions of circulation and respiration are 

 so intimately associated together, that it is impos- 

 sible to understand the modifications of the former 

 without taking into account its relations with the 

 latter. It will be necessary, therefore, in this place 

 to advert to the objects of this latter function, and 

 the mode in which the circulation contributes to 

 carry those objects into effect. 



The blood which has supplied the several organs 



