large lymphatic vessels into the great veins just at the entrance of the heart, and so the fat finally reaches the blood stream, but not so directly as do glucose and the digestion products of protein. There is a reason for this difference in the handling of fats and the other foodstuffs which will be clear when we recall the point made in the last chapter that all the blood which flows through the walls of the intestine is collected in the portal vein and must pass again through the capillaries of the liver before entering the circulation at large. What happens is that all the digestion products, except the fats, pass through the capillaries of the liver, but the fat gets into the blood stream by another way and does not go through the liver en route; for some reason or other it seems to be to the advantage of the body that the fat should not be allowed to circulate through the liver capillaries. It is, as we shall see in a later chapter, a distinct advantage to have the glucose pass through the liver, and there is probably also a good reason for having the digestion products of protein take that course. The digesting food is moved along the small intestine by contractions of its muscular wall, which travel along slowly in the form of a wave, very much as the wave of swallowing passes down the neck of a cow or horse. These waves do not occur regularly but only now and then; what happens is that several inches or two or three feet of the small intestine will be filled with a food mass which stays for a while at that place, digestion going on in it, and the digested food being absorbed through the wall. Both the digestion and the absorption are aided by a kind of churning motion made up of a series of contractions spaced about an inch apart along the part of the intestine where the food mass lies. These contractions form and disappear quite regularly at the rate of several a minute. They have the effect of squeezing the food mass rhythmically, but do not move it away from the place where it is lying. After this churning has been going on for a while it subsides, leaving that part of the intestine entirely quiet; then a contraction wave sets in at the end of the food mass toward the stomach and pushes the mass bodily along the intestine to a new section where the wave dies out and the churning motion begins again. This process is repeated with every food mass as it comes through from the stomach into the intestine until, finally, usually three or four hours after the first food has commenced to come into the intestine from the stomach, the whole mass has been propelled the length of the small intestine and what is left has been passed on through into the large intestine. The small intestine is about twenty-five feet long in man; flesh-eating animals of about the same size have intestines only about one-half as long; grazing animals the same size, on the other hand, have intestines two or three times longer