The organs specially devoted to absorption are the villi, which project into the contents of the small intestine. Each is a conical process about 0·5 millimetre long. The villi are longest in the upper half of the small intestine. Below this level they decrease in number and size. A villus is completely covered with epithelial cells of short, columnar form. The free border of each cell is slightly hardened, forming a disc or cap which appears striated in optical section—an indication, as some think, that it is traversed by pores. Others hold that the appearance of striation is due to minute cilia-like projections which beset the free border of each cell. In worms and other invertebrates the cells carry motile projections of not inconsiderable size, which no doubt free their surfaces from the unassimilable matter which tends to accumulate upon them. Possibly they help to fix particles which are suitable for absorption. In mammals the presence of cilia has not been demonstrated. The extreme minuteness of the striæ seems to point to their being merely indications that the border is permeable to fluids, including droplets of fat. The so-called basement membrane upon which the epithelial cells rest must not be regarded as a membrane in the physical sense. Rather is it a basket-work which supports the cells, without in any degree limiting their power of disgorging into the lymph-spaces of the villi the substances which they have absorbed. Within the villus, connective tissue forms a sponge-work, the spaces of which are filled with lymph, in which considerable number of leucocytes roam, on the look-out, no doubt, for any germs which may make their way between the epithelial cells. In the centre of the villus is a lymphatic radicle—_i.e._, a fusiform cul-de-sac—which is the dilated end of a lymph-vessel. It, like all other lymph-vessels, is walled by flattened endothelial scales. It communicates with the lymph-plexus beneath the mucous membrane, which, again, communicates with a coarser plexus outside the muscular coat. From the peri-intestinal plexus vessels lying in the mesentery converge to the receptaculum chyli, the bulbous commencement of the thoracic duct, which lies at the back of the abdomen in front of the bodies of the vertebræ. The thoracic duct runs up the front of the vertebral column, through the thorax, and then hooks over to pour the fluid which it conveys into the great veins shortly before they join the heart. After a meal containing fat the fluid in the lymphatic vessels of the mesentery, the lacteals, has, as already stated (p. 43), the appearance of milk. The fat absorbed by the epithelium covering a villus is passed on into its lymph-space. From this into the central lacteal receptacle, thence to the submucous and peri-intestinal plexuses, the lacteal vessels of the mesentery, the thoracic duct. Absorbed fat does not pass through the liver, but is carried into the heart; thence through the lungs, and back to the