in a continual change of matter in the parts already formed (the vegetative as well as the higher and properly animal parts), lives absolutely the life of the plant. We likewise observe, that here also the fluid parts are the sources from which the several solid parts are formed by different attractive affinities, for this reason, that the external nourishing substances first enter into the fluids, but at the same time the secretions which are given off from the fluid parts of the body are carried away in the form of liquids, vapours, or gases. However, to perform this attraction and repulsion, one condition is requisite which is equally necessary in the life of plants, and that is the circulation of the fluids. This circulation we have seen performed in the plant through the polarity between the root and the flower, and therefore in a linear direction from one end to the other. But since the animal is a plant the root of which is turned inwards, a similar motion of the fluids, from the absorbing to the exhaling pole, instead of being directed linearly from one extremity of the body to the other, must take a centrifugal and centripetal direction, inasmuch as it is but an action and reaction between external and internal organs. We have observed that the circulation of the fluids in the embryo is between the embryo as a centre and the integuments of the egg as the periphery; and that the circulation is not confined to the embryo itself until it has attained its full maturity[1], at which period it is observed alternating between the heart (as the centre of vegetation) and the periphery; and in the higher animals and man, partly toward the whole bodily structure and its common integument the epidermis, partly toward the epidermis turned inwards in the organ of respiration, and vice versâ. Hence it follows that this movement of the sap or blood is not properly a circular motion, and has therefore no true resemblance to the rotatory motion of the heavenly bodies; because these have an intermediate movement which is the result of attraction and repulsion; whereas the movement of the blood appears as an alternation of attraction and repulsion, which is most analogous to the ascending and descending movement of the sap in the plant. This also leads to the conclusion that both this movement and its direction are originally the mere effect of the polar attraction and repulsion of the fluid itself. Wherefore this circulation may take place without needing any other mechanical aid, as for instance the pressure of the vascular surfaces, of which aid many animals are for the most part destitute. Indeed, the original unimportance of such aids is rendered fully evident by the fact that the vessels, like all other solid parts, are formed only by the circulating fluids,—consequently, that
- ↑ The essential difference between the earliest and the later form of the circulation in the egg during the process of incubation has been recently demonstrated in the beautiful experiments of Pander.
