The smoothness of the free surface of the serous system greatly facilitates the motions of the organs which it covers. We have already observed that nature employs two principal means for this object, viz. the membranes and the cellular texture. By distributing externally the second of these means, it has designed the first especially for internal motions. The smoothness and moisture of the serous surfaces are singularly favourable for them. These internal motions are usually regarded only in an insulated manner, as relating to the functions of the organ that executes them, as in relation to the circulation for the heart, respiration for the lungs, digestion for the stomach, &c. But they should also be considered in a general manner; they should be regarded as carrying through the whole machine a continual excitement which supports and animates the forces and the action of all the organs of the head, the chest and the abdomen, which receive less sensibly than the organs of the extremities, the influence of external motions. It is these internal motions that excite, sustain, and develop within, the nutritive phenomena, as the motions of the thigh, the arm, &c. without, favour the nutrition of the muscles which are found there; this is seen very evidently in bakers and other mechanics who exert more particularly this or that part. It is thus that the serous membranes contribute indirectly to the nutrition and growth of their respective viscera; but they never have a direct influence upon this nutrition, because their organization and life are different from the life and organization of these viscera. The free surface of the serous system differs essentially from that of the mucous, in this, that it contracts frequent adhesions. The pleura is of all the serous organs, that in which these adhesions are the most evident. We find almost as many dead bodies in which they exist, as we do those in which they do not. Next to the pleura is the peritoneum, then the pericardium, then the tunica vaginalis, then the arachnoides, which is that of all the serous surfaces in which adhesions are the least frequent, though I have observed them in it. These adhesions exhibit many varieties which can be studied best on the pleura, which are as follows.