I. _General Division of the Capillaries._ Since this system is not destined merely to unite the arteries to the veins, and change the black blood to red, it is evident that other fluids besides the blood, must circulate in them; this is in fact what observation proves. There are many parts in the animal economy, in which white fluids alone circulate. We know the hypothetical opinions of Boerhaave upon the white arteries, the decreasing vessels, &c. We shall find these opinions in all books; I shall only say here what accurate observation shows us. That there is in the general capillary system, parts in which the blood especially moves, others pervaded only by white, or greyish fluids, &c. is a thing of inspection, and has no need of proofs. But what is the proportion of these fluids in the different organs? it is this that must be examined; now there are parts where the blood predominates almost exclusively in the capillary system, others where it exists in part, and in which there is a portion of different fluids, others in fine in which these fluids alone are found. _Of the Organs in which the Capillaries contain only Blood._ It appears that in the muscular system, in the spleen, in certain parts of the mucous surfaces, as in the pituitary membrane, &c. the blood so predominates in the capillary tubes, that almost every other fluid is unknown in them; thus fine injections demonstrate few other vessels; the arteries and the veins are seen there in great abundance. The blood, or its colouring matter, is in them, as I shall say, in two different states; in one, it stagnates and serves then to colour the organ; in the other, it circulates and contributes to its nutrition, its excitement, &c. _Of the Organs in which the Capillaries contain Blood and Fluids differing from it._ These organs are the most numerous in the animal economy. The bones, the cellular texture, the serous membranes, a part of the fibrous system, the skin, the vascular parietes, the glands, &c. &c. exhibit this arrangement in a very remarkable manner.