ABSORPTION AND ASSIMILATION 91 



blood tube of the villus, while its greater part enters 

 the lacteal tube. These lacteals unite to form larger and 

 larger tubes, which run across the mesentery, and finally 

 open into a single tube, the thoracic duct, running up the 

 spinal column. This is a tube as large as a goose quill, 

 and opens into a large vein at the root of the neck, where 

 emulsified fat from the intestine first reaches the blood. 



124. Completion of digestion. Reckoning the amount of 

 saliva as two pints a day, of gastric juice as eight pints, of pancreatic 

 juice one and a half pints, and bile as two pints, and of food three pints, 

 the liquid introduced into the intestine daily amounts to two gallons at 

 least, and nearly the same amount is absorbed. More and more of this 

 liquid is absorbed as the food passes down the intestine, until, about 

 twelve hours after eating, what is left of the food and digestive fluids 

 reaches the large intestine in a semi-solid state. In the large intestine 

 absorption and peristalsis are so very much slower, that from twenty- 

 four to thirty-six hours are required for the remains of food to traverse 

 it. Its water and digested food and some of the bile are absorbed, 

 while the rest of the bile and its other waste products and undigested 

 matter are left behind. In health the intestine expels the waste matter 

 regularly at least once a day. 



125. Assimilation of fat. Changing the digested food 

 into the various fluids and tissues of the body is assimila- 

 tion. The thoracic duct pours the digested fat into a large 

 vein on the left side of the neck, whence it is carried with 

 the venous blood to the lungs. Little or no fat can be 

 found in the blood leaving the lungs unless it has been 

 eaten in excessive quantities. It is probably oxidized at 

 once to carbonic acid and water, an ounce requiring three 

 ounces of oxygen. It is unlikely that any fat from the 

 food is stored up, but the fat in the body is probably 

 derived from the albumin of the cells. The oxidation of 

 fat produces heat, and the heat may be changed to power, 

 or be used simply to warm the body. 



