_Nuck_ hath sometime ago demonstrated, that these conglomerate Glands are a Bundle of small Vessels; that their excretory Ducts are Continuations of the arterial Ramifications, and that these Glands owe their Origin to the smallest Branches of the Arteries: These Arteries, which enter the glandular Substance of the Breasts, are imperceptible to the naked Eye, and discoverable only by injecting a very fine Tincture (which _Nuck_ tells us is known to very few Anatomists) into the Artery; this may be so far propelled, as to render the milky Ducts conspicuous. For the better Discovery of this Matter, _Nuck_ instituted another Experiment equally curious and useful; having met a Nipple full of excretory Ducts, he pressed it, and the Breast adjoining, so as to empty all its Contents, and having pitched upon one of the widest Ducts, he injected _Mercury_ so artificially, that he immediately observed the milky Ducts spread like Branchings of Trees; some Part of the _Mercury_ was carried so far as to enter the Arteries, whence the milky Vessels were continued.
Hence it follows, that these milky Ducts are destitute of Valves, otherwise the _Mercury_ and the injected Liquors would have been obstructed in their Passage. It is indeed observable, that these Canals are in some Places streighter and narrower than in others, so as to give some kind of Obstacle to the Injection; this is not to be imputed to Valves, but to some kind of Hardness peculiar to the Substance of the Glands, by which the milky Vessels are compressed. From hence appears the immediate Inosculation of the milky Ducts, with the small Ramifications of the Arteries, of which these Glands are composed; so that the arterial Blood propels and deposits its chylous and serous Particles by gentle Pressure and Impulse in the milky Ducts, without other Mechanism than bare Straining and Secretion. For the further and more exact Description of these Ducts, see _Nuck’s Adenographia_.
It remains now to examine, of what kind of Particles chiefly Milk is composed; which appear to be these three: The first is a fat, butyraceous, oily, and sulphureous Substance. The second is cheesy, earthy, chalky, and saline. The third is the Vehicle of these, _viz._ serous, which is watry, with a Mixture of nitrous Salts. But these Parts don’t hold the same Proportion in the Milk of all Animals; Cows Milk is most used in Food, it is thick and fat, and contains more Butter than the Milk of other Animals; upon which account it nourishes more, and is more agreeable to the human Body. Ews Milk hath more earthy and cheesy Particles; Goats Milk is in a Mean between these two, only that its _Serum_ contains more of a nitrous Salt; whence _Etmuller_ conjectures, that it hath all the Virtues of Whey made from Cows Milk, especially in Heats and scorbutick Cases. Asses Milk is of all the thinnest, next to human; the Milk of other Animals, as not so usually brought into Food, I forbear to describe.