The circulation in the greater number of animals, however, is a more complicated pro cess than that which has just been described ; it consists, in fact, of two parts perfectly dis tinct from each other ; one whereby the blood is exposed to the action of the air in the appa ratus which, in connexion with the respiratory process, we have denominated lungs, gills, &c., another by which it is finally distributed for the uses of the system. This double circula tion is accomplished by a great variety of con trivances (vide articles HEART and CIRCULA TION). In some tribes we find more than one vessel,—two, or three, each apparently inde pendent of the other, though p together, which are subservient to the distri bution of the nutrient fluid to the different parts of the body of the animal.
The chief differences between vegetables and animals with respect to their circulation, con sequently, appear to be these : in vegetables the motion of the sap or aliment takes place through the whole of one of the tissues of which they consist ; that of the cambium or proper nutritive fluid through the whole of another of these tissues, in opposite directions simply, and by the intermedium of fascicu lated, very numerous, and independent vessels ; whereas the aliment of animals does not cir culate through their bodies, but the nutritive fluid prepared from it is collected and con fined within peculiar channels, connected at both extremities in such wise as to form a con tinuous circle. In vegetables we perceive nothing like tendency towards or distribution from a central reservoir, nothing like ramifica tion from larger to smaller branches, &c.; con sequently nothing like a heart, as we do in animals above the very lowest. In vegetables, again, we see nothing like the two-fold distri bution of the nutrient fluid within different orders of vessels, the one to the organs of respiration, the other to the system at large, as occurs among all animals possessing a some what complicated organization.
We have recognized the heart as the princi pal cause of the motions performed by the fluids within the bodies of animals ; but as neither all animals have a heart and yet exhibit their nutrient fluids in motion ; indeed, as a distinct circulation of the blood may be demonstrated in many animals, and probably takes place in all at periods of their evolution anterior to the existence of a heart ; and further, as vegetables exhibit a motion or circulation of their fluids without the agency of any special organ, it is necessary to acknowledge a new law by virtue of which the fluids of organized beings generally go their round or reach their destination. This law has been designated as
the propulsive,—a power inherent in the nu tritive globules of living beings, and one of the special laws superadded to the general and all-pervading forces that regulate the universe.
One fundamental distinction between the bodies of the organic and inorganic kingdoms we have found based upon the permanence of the parts, the constancy of the relations, affi nities, &c. of the component elements of the one, and the incessant changes or renewals and decompositions which these parts or elements undergo in the other. The various processes by which the aliment of vegetables and animals is converted into a succus proprius, the final means of their individual conservation and evolution we have now examined ; we have only farther to discover this nutrient juice con verted into the different tissues and substances of which organized beings consist, to have a complete view of the vital act of nutrition. But here we are compelled to pause. Of the processes by which this transformation is ac complished we know next to nothing; all we are assured of is, that each tissue and organ seizes upon and converts into its proper sub stance those particles enveloped in the general mass of circulating fluids brought into rela tionship with it, and which are adapted to this purpose, at the same time that the particles which have already been consolidated and served their office are reduced to the fluid state, absorbed back into the torrent of the circulation, and afterwards either abstracted and thrown out of the body by the operation of certain organs charged with this duty, or being subjected to the action of the atmos pheric air in the lungs, gills, skin, &c. are restored to their fitness once more to enter as temporary constituents of the organization. It is evident, therefore, that we are only ac quainted with this operation in its effects. The act of ultimate nutrition has been happily entitled one of continuous generation in each living being and its parts ; it takes place in conformity with the laws of vitality instituted, and probably originating and ending in living organized beings.