Organs Of Digestion, Circulation, And Respiration
( Originally Published 1904 )
One who has the happy faculty of finding the charm of romance in what too many people call the "dry facts" of science, will find the miracles of childhood's fairy stories altogether tame as compared with the narrative that tells us how the food that we eat is converted into living flesh.
For the layman—for him who cares not for the technicalities, and who is content with the results alone—it is a simple tale and easily told. Leaving out the detailed description of the finer parts of the mechanism with which Nature has endowed the organs, the process of making food over into living tissue in the human body may be briefly related as follows.
The change that the food undergoes begins right in the mouth, in which are a number of glands that supply saliva whenever food is chewed. As this saliva must be thoroughly mixed with the food if the best vitalizing results are to be obtained, thorough mastication is needed-at the outset. The most important . work that the saliva has to do is to convert the starchy portions of the food into sugar—not the sugar that we find in the dining-table bowl, but a form of it nevertheless. This is because sugar will dissolve in water and starch will not. Hence, as water is the fluid that- is supplied to the body, if the starch were not changed into a soluble sugar it would be compelled to leave the body as it entered it, and no nourishment could result from it in consequence.
No other part. of the food is altered by the saliva. If water be drunk-with meals to wash down improperly masticated food it interferes with the action of the saliva and also the digestive juices of the stomach.
From the mouth the food passes into the pharynx, a little sac that empties into the gullet, or esophagus. This latter tube empties directly into the stomach through the cardiac opening at the left side of the organ, while at its right side is located the pyloric opening. Both connect with the stomach on the upper edge.
Now the four coats of the stomach are most abundantly supplied with involuntary muscles. When empty, the coats of the stomach lie in folds, but as soon as food enters it these coats immediately unfold to give accommodation to the food. And at once the involuntary muscles begin to work, churning the whole stomach and causing the food to move about in a very lively manner. Both the cardiac and pyloric openings are closed except when their opening is necessary for the passage of food.
The churning that takes place mixes and mingles the food most thoroughly, especially if it has been well chewed by the teeth. The gastric juice is supplied from glands that line the innermost coat of the stomach. Interspersed between the glands are a myriad of tiny blood vessels. The inside of the stomach is dull, almost colorless when empty. But as soon-as food enters, blood hastens to it from all parts Of the body—not in a helter-skelter way, but through the regular blood channels, and in a highly orderly manner. The blood is "hungry" so to speak. As soon as the work of digestion has gotten well under way, the blood vessels begin to absorb nourishing particles of digested matter through the thin membranes that separate them from the contents of the stomach.
This digested matter, which becomes of a dull grayish color and is called chyme, is the product of the action of the gastric juice on the food in the stomach.
In this juice are pepsin and rennet. The rennet by the aid of the free hydrochloric acid that is found in the stomach, dissolves all of the proteid elements that exist in the food. The proteids are substances that contain nitrogen, and are necessary to the maintaining of life. Wheat is a sample of a proteid food; so are peas and beans, lentils and cereals. The fats are not dissolved in the stomach, but their tiny particles are set free in globules, and pass on to the small intestine, there to be acted upon further.
After the digestive process has been carried as far as it can be in the stomach, the food passes on to the small intestine, some twenty feet of which lie coiled in the abdomen. Here the fat is attacked by the intestinal juice and is rendered fit for the blood. The food passes continuously through the small intestine, impelled by the involuntary contractions of the muscles of that organ. All that the mouth and the stomach have left undone in the way of digestion is completed in the small intestine, which is lined with tiny blood vessels that absorb whatever is needed of nutriment from the food fluid.
And then what is left of the food—mainly offal, refuse—passes into the large intestine, composed of the ascending, transverse and descending colons. Along these three parts the blood absorbs little, if any, remaining nourishment, and the waste matter is expelled from the body through the descending colon.
It is the function of the liver to add bile, and of the pancreas to supply pancreatic juice to the contents of the small intestine. The result is that a milky fluid known as chyle is separated in the small intestine from the chyme. This chyle is absorbed by the lacteal vessels and through them is assimilated with the-blood. In addition, the liver, through having a very important system of blood vessels, acts as an excreting organ in removing many dead and poisonous matters from the blood.
Now the blood—which, bear in mind, is the vitalizing fluid of the whole body is pumped through the arteries by the ever-busy heart. It is the work of. the arteries to carry the enriched blood through the body. These arteries are everywhere dividing and subdiwiding, and becoming smaller and smaller with- each new subdivision until their course is lost to. the naked eye.
Everywhere that the hungry cells are crying for new nourishment they seize it from the fresh, pure blood that is coming to them. And everywhere that new, good material is left by the blood, the old, dead cell matter is taken up by it in exchange. The oxygen that is mingled with the blood burns up much of this waste matter, which is found mainly in the form of carbon.
And when the blood has reached the limits of the arteries it is soaked up greedily by the capillaries—tiny, hair-like canals that pass between the arteries and the veins. And these capillaries give out much of the'nourishment that is left in the blood to the cells that want it.
So through the capillaries the impure blood is carried into the veins, whose mission it is to carry the bad, or venous, blood back to the heart. All through the body the blood is also enriched as is necessary from the lymphatics, a system of vessels that carry the lymph. Now, this lymph is very much like blood itself, except that it does not contain red corpuscles. Acting with the lymphatics are the lacteals, and the lacteal fluid is very like lymph and is used for the enrichment of the blood. Thus we understand that, at whatever point the blood needs enrichment, Nature has provided the means; man has only to provide the supply through food. But he must be sure that his food contains the needed elements.
But the venous blood comes back to the heart in a befouled condition. It flows into the upper chamber (auricle) of the right side of the heart, is forced down to the lower chamber (ventricle). and thence to the lungs. Here the important work of purification goes on. And how? By fresh, sweet air.
Air that is breathed in passes down through the trachea or windpipe, which is subdivided into two tubes or bronchi, one passing to the right lung and one to the left. These are again subdivided into a great many bronchial tubes that reach all parts of the lungs, becoming at last very fine indeed. At the end of each of these very tiny tubes is a small, bell-like opening that is known as an alveolar cell—although it is not a cell at all in the strict anatomical sense of the word.
Now, through these cells the inhaled air passes through the very thin membranes of the blood vessels in the lungs. Wherever a bit of dead (poisonous) carbon is found in the blood, the oxygen of the air in the --lungs burns it up, forming carbonic: acid gas, which, together with surplus moisture in the blood, is expelled from the body by the exhalation of the breath. Dust and other irritating foreign substances that are inhaled may be gotten rid of by the act of coughing.
And now, when the blood in the lungs has been cleansed of its impurities, it passes on to the left auricle of the heart, thence down into the left ventricle, and is pumped once more through the body on its round of revitalizing all of the tissues, even to those of the bones—for bones must be fed as well as flesh..
Thus we have traced the digestion of food, the work of the blood and its ultimate purification before being used over again. It will. be seen that blood cannot be purified unless sufficient quantities of absolutely unpolluted air are taken into the lungs. Hence the necessity for breathing exercises which enable the lungs to attain their ultimate power for good by developing their air-capacity to the utmost. Health, without an ample supply of oxygen for the body through the lungs, is out of the question.