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.
[52.] This, then, is what becomes of the food-stuffs:—
The fats are broken up by the bile and pancreatic juice into minute particles. These minute particles, we do not exactly know how, pass through the epithelium of the villus into the lacteal vessels, from the lacteals into the thoracic duct, and from the thoracic duct into the vena cava. Thus the fats we eat get into the blood.
The starch is changed into sugar in the mouth by saliva, and in the intestine by the pancreatic juice; but sugar passes readily through membranes, and so slips into the blood capillaries of the walls of the alimentary canal. Thus all the sugar we eat, and all the goodness of the starch we eat, pass into the blood.
The proteids are dissolved in the stomach by the gastric juice, and what passes the stomach is dissolved in the intestine, dissolved in such a way that it can pass through membranes; and thus proteids pass into the blood.
Probably some of the sugar and proteids pass into the lacteals as well.
The minerals are dissolved either in the mouth, or in the stomach, or in the intestine, and pass into the blood.
And water passes into the blood everywhere along the whole length of the canal.