2d. The internal absorptions, on the contrary, every where correspond with the analogous exhalations. Thus the absorbents take up, upon the serous system, the serum, upon the cellular system the serum and fat, upon the medullary system the marrow, upon the synovial system, the synovia; fluids, all of which as we have seen, had been thrown out by exhalation upon their respective surfaces, and had remained for an instant upon them. These absorptions are made in a constant and regular manner; it is in this that they differ from the preceding. The internal absorbents, incessantly in action, take up in a given time the same quantity of fluids; their action corresponds precisely with that of the exhalants. Observe that there are two essential differences between the external and internal absorptions; viz. that the one are exerted on the one hand on fluids different from those exhaled upon their surfaces, and that on the other they are subject to continual variations and irregularities; whilst the others always take up the fluids exhaled upon their surfaces, and are constant and regular, at least in a state of health. I shall point out in the mucous and cutaneous, systems, the cause of this important difference. 3d. As to the nutritive absorptions, we know much less of them than the preceding; but nutrition evidently supposes them. There is in fact in this function a double motion, one of composition, the other of decomposition. No organ, no part of an organ is formed at one period of the same elements that composed it at a former one. The ancients thought, without positive proof, that the body was renewed every seven years. Whatever may be the period of its renewal, we cannot deny that it is continually composed and decomposed; now the exhalants perform the first nutritive motion, the absorbents the second. Observe in fact, that the internal substances never re-enter the circulation to be afterwards thrown out, except by the way of the absorbents. The nutritive absorptions differ then from the preceding, in this, that the substance deposited by exhalation and taken up by them, remains in the organs, makes a part of them, and contributes to their composition; whilst the fluids with which the internal exhalations and absorptions are concerned, after having been furnished by the one, and before being taken up by the other, remain out of the organs, upon their surface or in their cells, but do not make a part of their structure.