The Emulsifying Action of the Bile - The Absorption of Digested Foods - The Work of the

Blood - The Function of the Liver

From what has already been said of digestion in the mouth and stomach it can readily be understood that digestion is a complicated process divided into several stages. We have considered the first two stages in the previous nursing article, and we have reached the stage when the food leaves the stomach and enters the small intestine. It is now acted upon by the juices of the intestine, the bile from the liver, and the pancreatic juice secreted by the pancreas. Up to the present time the fatty matters in the food have not been dealt with at all. They undergo no change in the mouth and stomach.

As soon as they pass into the intestine, forming part of the chyme from the stomach, they met t the bile, which is a golden brown fluid secreted by the liver, a large organ situated on the right side of the abdomen. The action of the bile on the fatty matters in the food is to emulsify them - i.e., the large globules of oil are broken up into very tiny globules or drops so small that they can pass through the living membrane. This emulsifying action of the bile is due to the presence of soda salts, and a simple experiment which anyone can try will give a practical illustration of what takes place. Take half

Medical a tumblerful of water, and add to it a table-spoonful of oil. It is impossible by any amount of shaking to make these two substances blend together. Now add some soda to them, or half a teaspoonful of borax, which is in reality bi-borate of soda, and shake. The oil in the water becomes miscible because the oil globules are broken up, and the result is emulsion. If, instead of soda or borax, some bile were added to the oil and water, the same result would follow.

The next important juice in the small intestine is the pancreatic juice, which is secreted by the pancreas, or sweetbread, a gland that lies underneath the stomach. This juice has a very complex function. By means of its ferments it can:

1. Change starch into sugar.

2. Emulsify oils and fats.

3. Convert insoluble, albuminous substances or proteids into soluble peptones.

This threefold action of the pancreas is very useful. It can deal with any starch that has escaped conversion into sugar in the mouth. It can change any proteids into peptones which have been neglected by the stomach. It helps the bile to emulsify the fats.

The third juice in the intestine is called the intestinal juice, because it is secreted by the glands in the walls of the intestines, and it acts upon the starches and proteids.

As a result of the bile, pancreatic and intestinal juices acting upon the chyme, it is gradually converted into a yellowish white fluid called chyle. This is absorbed. The remaining undigested parts of the food are passed along the intestines and finally expelled from the body.

Now, what happens to this chyle, with its rich store of nourishment derived from the meals we eat, after it has gone through the complicated process of digestion?

It must, of course, be "absorbed" from the digestive canal before it can be sent to other parts of the body.

The Absorption Of Digested Pood

The first thing a student of digestion has to understand is that the whole digestive canal is lined with a thin, smooth membrane such as lines the mouth and covers the inner aspect of the lips. This mucous membrane is full of invisible glands and invisible blood-vessels, which lie under one single layer of cells which are microscopic in size. Thus the food-stuffs lie against the fine capillaries, or minute bloodvessels, separated only by a thin, transparent wall of membrane. These fluid substances can, therefore, pass through this membrane, through the walls of the tiny blood-vessels, right into the blood. The capillaries of the stomach become very active or congested during digestion, and absorb, or take up, sugars and soluble peptones. A small amount of absorption even takes place in the mouth. A still greater amount of nourishment is absorbed from the stomach.

Now we come to the intestines, which are very much concerned in the absorption of food.

The mucous membrane of the small intestine is specially constructed so as to increase its absorptive surface. It contains innumerable tiny projections called "villi," which is the Latin word for "small hairs." If the finger is drawn over the interior of the small intestine, the surface feels like velvet, because of the presence of these minute villi. Inside these villi is a network of small capillaries and another network of vessels called lacteals. Now, the network of blood-vessels is actively engaged in absorbing peptones and sugars, whilst the lacteals are reserved for the absorption of the emulsified fats. These lacteals are part of a vast system of vessels throughout the body which we have not yet mentioned, called "lymphatics."

The lymphatic vessels contain a milky fluid called lymph, and one of the functions of the lymphatic glands is to filter poisons, and also to produce white corpuscles for the blood. An excellent example of how lymphatic glands act as a filter is provided by the instance of a poisoned finger being accompanied by enlarged glands in the arm or armpit, when the microbes are filtered by the glands of these situations. The "lacteals" are really lymphatics in the small intestine, and this name is given to them because during digestion they are filled with milky (lacteus-milk) fluid, absorbed from chyle, which is the fat.

The digested food, therefore, finds its way into the blood by two channels:

1. The sugars, peptones, mineral salts, or salines, and water, pass directly "into the small blood-vessels of the stomach and intestines.

2. The fats are absorbed by the lacteals in the villi of the small intestines.

Now, how do the digested fats get into the blood ultimately? The small lacteal vessels gradually increase in size and unite, forming a sort of network. These lacteals go on uniting until they open into the thoracic duct, a long tube passing up the thorax, or chest, and along the side of the vertebral column, which ultimately opens into the large veins just before these enter the heart. The digested fat passes from the intestines to the lacteals, then into the thoracic duct, and then into the blood. Now let us consider the capillaries of the intestine. These little blood-vessels unite and become larger and larger until they are one large vein, called the portal vein. The function of this portal vein is to collect the blood from the intestines and stomach, and to carry it to the liver.

This blood is rich in sugar and peptones and salts obtained from the intestines. Nature does not wish this rich food-stuff to be carried straight away to the different organs and then to be soon eliminated from the body by the excretory organs such as the kidneys and lungs. If things were arranged in this way we should be needing fresh supplies of food constantly, and would have to spend our lives eating or stoking like an engine with its constant supply of fresh fuel. So the liver is provided as a sort of store cupboard. Part of the sugars and albumens are taken by the portal vein to the liver cells to be stored.

It will be seen from this that the liver has at least two functions:

1. To secrete bile for the digestion of fats.

2. To act as a great store cupboard for the food which we require whilst we sleep or work, to save us the inconvenience of constantly eating.

The stored sugar and stored albumen are slowly sent up into the blood from the liver as they are required by the body.

Now we have finished the story of the digestive process. You have learned how our daily meals actually produce materials for the blood. In the article on the circulation it was demonstrated how this blood was taken to the heart and then sent to the lungs for a supply of oxygen, which it received from the air we breathe. The oxygenated blood was then pumped by

Medical the heart to every point of the body. Aerated blood goes to the muscles, which derive energy from the blood to do their work of locomotion. It goes to the skin, and keeps it healthy and "alive." It goes to the brain and nourishes the nerve cells, where consciousness lies, where ideas are generated, where abstract thought is evolved.

The lesson to be derived from the description of digestion is that the human body is a selfrepining machine. If we supply this machine with the right sort of fuel or food at the proper intervals of time it will have its- due supplv of energy for work. If we, in ignorance or from deliberate foolhardiness, ignore the needs of the digestive system, sickness and ill-health will inevitably follow.

In the next article we shall consider the physiology of the nervous system, and study the functions of the brain.