Of these, we shall first consider that of the late famous professor Boerhaave. He observed, that every artery of the body is greater, in its diameter, than any of its branches: and this observation being found true, as far as our eye and the microscope can inform us, he inferred, by analogy, that it held good even thro’ the most minute subdivisions of the arterial system. But, says he, proportionable to the diameter of the canal is the size of the particles moving thro’ it: therefore, if an ultimate capillary artery, admitting only one red globule at once to pass thro’ it, send off lateral branches, these branches will be capable of receiving such particles only as are smaller than a red globule. But the particles next in magnitude below the red globules are the yellow serous ones; and the lateral vessel, thus receiving them, is a serous artery, and the trunk of a second order of vessels. In like manner, this trunk, being continued on thro’ many lessening branches, will at last grow so minute, as to admit only one serous globule: its lateral branches, therefore, will receive only such particles as are smaller than the serous ones: but these are the particles of the lymph; and this lateral branch is a lymphatic artery, and the trunk of a third order of vessels. Thus, in the red arteries are contained all the circulated fluids of the body; in the serous arteries, all except the red blood; in the lymphatics, all except the red blood and serum: and this subordination is, according to the same laws, continued down thro’ fluids more subtile than the lymph, to the smallest vessel, which is propagated from the aorta. Such was Boerhaave’s doctrine concerning the vascular system of animal bodies; like many of his other notions, ingenious, plausible, and recommending itself, at first sight, by an appearance of geometrical and mechanical accuracy: but founded upon insufficient data, and by no means to be reconciled to appearances.