Our lesson on digestion made clear to us the fact that all food, if it is to be of any service to us, must enter the stream of the blood, and it can only do so by osmosis through the walls of the blood-vessels themselves. We learned further that only dissolved substances are capable of passing through a membrane in this way, hence the whole purpose of digestion. Digestion is simply the work of so dissolving the food as to render it osmotic.
Starch is insoluble, and therefore has no osmotic force. The starchy matters of the food are converted into sugar by the action of the saliva; sugar is soluble and, of course, readily osmotic, and, as the food-stream passes along the alimentary canal, this dissolved sugar is greedily sucked up by direct absorption into the network of capillary vessels, which closely line its walls.
It is exactly the same with the proteid matters of the food. The lean parts of meat, together with what is left of the bread and pudding, peas and beans, after the starch has been removed, form a substance resembling albumen or white of egg. If some "white of egg" were put into a bladder and immersed in a vessel of water, it would beseen that no exchange takes place. The substance cannot pass through a membrane in that state. It is the work of the gastric juice to dissolve all these proteid matters, and, when dissolved, they are readily taken in, by direct absorption, through the walls of the blood-vessels.
It still remains to account for the fatty matters of thefood. They leave the stomach in an undissolved state, for neither the saliva nor the gastric juice has any effect upon them. They can be dissolved only by an alkaline fluid; this the liver supplies in the shape of bile. It is the bile that dissolves the fatty, oily portions of the food, and so renders them osmotic.
But the food-stream, as it passes out from the stomach into the intestines, contains not only these undissolved fats for the bile to act upon; some of the starches and proteids remain undissolved. This explains why, side by side with the bile duct, another little channel opens into the intestine,bringing from the pancreas a fourth dissolving fluid, known as the pancreatic juice. This fluid combines in itself the properties of both saliva and gastric juice, and completes the work upon all starches and proteids which have escaped the action of those juices in the stomach. As these substances are dissolved, and thus rendered osmotic, they are at once sucked up into the blood, as before, by direct absorption, all along the intestinal canal.
But the intestines themselves have a wonderful part to play in the absorption of the dissolved food. The innersurface of the small intestines is not smooth, like the outside of the tube, but is covered with a multitude of tiny thread-like projections, set close side by aide, like the pile of velvet. Itis estimated that there are no less than four million of those little hanging threads in the walls of the small intestines. They are called villi. They might be compared to a multitude of tiny tongueshanging down into the tube.
They are being constantly bathed by the stream of dissolved food as it passes along the intestinal canal. These little tongues suck up from the stream the dissolved fats which it contains.
Each little villus, small as it is, is really a very complicated structure. It has, running through its center, a tiny tube, which forms at its extremity an exceedingly fine network of still smaller tubes. This tube is not for the purpose of carrying blood. It is called a lacteal. The name lacteal comes from the Latin word lac, meaning milk. It signifies milky-looking. The name is given to these vessels becauseof their white, milky appearance an hour or so after ameal. This white, milky appearance is actually due to the minute particles of fat, which they have absorbed from the intestines after the bile has dissolved and broken them up.
The work of these tiny tubes, the lacteals, is to absorb the digested fatty parts of the food. All the fat which enters the blood is carried to it through the lacteals. They gradually unite into larger vessels in the walls of the intestines, and these at last form a great trunk or pouch called the thoracic duct.
The thoracic duct is a tube about 18 inches long, and about the size of an ordinary goose-quill. It lies along the front of the spinal column, and extends upwards to the root of the neck. It forms a sort of reservoir or receptacle for the dissolved fats, which are sucked up by the lactealsand poured into it. At its upper extremity it opens into a great blood-vessel known as the left sub-clavian vein. Thus, the fatty matters, taken up by the lacteals, at last reach the blood, and the work of digestion is completed.
