the intestine. These are called =villi=; they are not unlike the papillæ of the skin (Fig. 15), if you suppose all the epidermis stripped except the bottom row of cells (_d_), and the papilla itself pulled out a good deal. Fig. 18 is a sketch to illustrate the structure of a =villus=. The epithelium (_b_), you see, is made up of a single row of cells. Beneath the epithelium, just as in the papilla of the skin, is a network of blood capillaries, shown, for convenience, in the right-hand villus only. But besides the blood capillaries, there is in each villus, what there is not in a papilla of the skin, another capillary (shown, for convenience, in the left-hand villus only) which does not contain blood, which is not connected with any artery or with any vein, but which begins in the villus. This is a =lacteal=. I have said nothing of these at present. In most parts of the body we find, besides blood capillaries, fine passages very much like capillaries, except that they contain a colourless fluid instead of blood, and do not branch off from any larger vessels like arteries. They seem to start out of the part in which they are found, like the roots of a plant in the soil. But though unlike blood capillaries in not branching off from larger trunks, they resemble capillaries in joining together to form larger trunks corresponding to veins, and the colourless fluid flows from the fine capillary channels towards these larger trunks. This colourless fluid is called =lymph=; it is very much like blood without the red corpuscles, and the channels in which it flows are called =lymphatics=. The lymphatics from nearly all parts of the body join at last into a great trunk called the =thoracic duct=, which empties itself into the great veins of the neck, as is shown in the diagram, Fig. 6, _Lct._, _Ly._, _Th. D._ Now, many of the lymphatics start from the innumerable villi of the intestine, and are there called =lacteals= (Fig. 6, _Lct._); so that lacteals may be said to be those lymphatics which have their roots in the villi of the intestine. But what has all this to do with the digestion of fat? Lacteal means =milky=, and the lymphatics coming from the villi are called lacteals because, when digestion is going on, the fluid in them, instead of being transparent as in the rest of the lymphatics, =is white and milky=. Why is it thus white and milky? Because it is crowded with minute particles of fat, and those minute particles of fat come from the inside of the intestine. They are the same minute particles into which the bile and pancreatic juice have divided the fat taken as food. We know this because when no fat is eaten the lacteals do not get milky; and when for any reason bile and pancreatic juice are prevented from getting into the intestine, though ever so much fat be eaten, it does not get into the lacteals at all, it remains in the intestine in great pieces, and is finally cast out as useless.