What is the structure of the capillaries? So great is their tenuity that we evidently cannot have upon this point, any kind of data founded upon experiment and observation. Only it is very probable, it is even certain, that this structure is modified differently in each organ, that it is not the same in the tendons, the aponeuroses, the muscles, &c. that it really partakes of the nature of the organ of which it makes an integral part. The membrane which lines the excretories, the arteries, the veins, the exhalants, vessels which go into the system of the capillaries or come out of it, is very like that of these capillaries; but it is not certainly the same. It is the diversity in the structure of the capillaries, according to the organs in which they are found, which has an essential influence upon the difference which the vital properties exhibit, particularly the organic sensibility and the insensible organic contractility in each system in which we examine them; hence peculiar modifications in all those diseases over which these properties preside, and which are seated especially in the capillaries, such as inflammations, tumours, hemorrhages, &c. &c. The difference in structure of the capillary system, sometimes becomes manifest to the eye. Thus the spleen, the corpus cavernosum, instead of presenting, like the serous surfaces, a vascular net-work in which the blood oscillates in different directions, according to the motion it receives, exhibit only spongy, cancellated textures, whose nature is but little known, in which the blood appears often to stagnate, instead of moving, &c. VIII. _Of the Circulation of the Capillaries._ The circulatory phenomena are of two kinds in the capillary system: 1st, there is the motion of the fluids; 2d, the alterations which they undergo. _Motion of the Fluids in the Capillary System._ These fluids are, 1st, the blood; 2d, others differing from it in their composition, though we only know their differences of appearance. Let us examine the laws of the motion of each kind.