Position and Structure of the Pancreas.--The most important digestive gland in the human body is the pancreas. The gland is a rather diffuse structure; its duct empties by a common opening with the bile duct into the small intestine, a short distance below the pylorus. In internal structure, the pancreas resembles the salivary glands.
[Illustration: Appearance of milk under the microscope, showing the natural grouping of the fat globules. In the circle a single group is highly magnified. Milk is one form of an emulsion. (S. M. Babcock, Wis. Bul. No.
61.)]
Work done by the Pancreas.--Starch paste added to artificial pancreatic fluid and kept at blood heat is soon changed to sugar. Protein, under the same conditions, is changed to a peptone. Fats, which so far have been unchanged except to be melted by the heat of the body, are changed by the action of the pancreas into a form which can pass through the walls of the food tube. If we test pancreatic fluid, we find it strongly _alkaline_ in its reaction. If two test tubes, one containing olive oil and water, the other olive oil and a weak solution of caustic soda, an _alkali_, be shaken violently and then allowed to stand, the oil and water will quickly separate, while the oil, caustic soda, and water will remain for some time in a milky _emulsion_. If this emulsion be examined under the microscope, it will be found to be made of millions of little droplets of fat, floating in the liquid. The presence of the caustic soda helped the forming of the emulsion. Pancreatic fluid similarly emulsifies fats and changes them into soft soaps and fatty acids. Fat in this form may be absorbed. The process of this transformation is not well understood.
Conditions under which the Pancreas does its Work.--The secretion from this gland seems to be influenced by the overflow of acid material from the stomach. This acid, on striking the lining of the small intestine, causes the formation in its walls of a substance known as _secretin_. This secretin reaches the blood and seems to stimulate all the glands pouring fluid into the intestine to do more work. A pint or more of pancreatic fluid is secreted every day.
The Intestinal Fluid.--Three different pancreatic enzymes do the work of digestion, one acting on starch, another on protein, and a third on fats.
It has been found that some of these enzymes will not do their work unless aided by the _intestinal_ fluid, a secretion formed in glands in the walls of the small intestine. This fluid, though not much is known about it, is believed to play an important part in the digestion of all kinds of foods left undigested in the small intestine.
Liver.--The liver is the largest gland in the body. In man, it hangs just below the diaphragm, a little to the right side of the body. During life, its color is deep red. It is divided into three lobes, between two of which is found the _gall bladder_, a thin-walled sac which holds the _bile_, a secretion of the liver. Bile is a strongly alkaline fluid of greenish color. It reaches the intestine through the same opening as the pancreatic fluid. Almost one quart of bile is passed daily into the digestive canal.
The color of bile is due to certain waste substances which come from the destruction of worn-out red corpuscles of the blood. This destruction takes place in the liver.
[Illustration: Diagram of a bit of the wall of the small intestine, greatly magnified, _a_, mouths of intestinal glands; _b_, villus cut lengthwise to show blood vessels and lacteal (in center); _e_, lacteal sending branches to other villi; _i_, intestinal glands; _m_, artery; _v_, vein; _l_, _t_, muscular coats of intestine wall.]
Functions of Bile.--The action of bile is not very well known. It has the very important faculty of aiding the pancreatic fluid in digestion, though alone it has slight if any digestive power. Certain substances in the bile aid especially in the absorption of fats. Bile seems to be mostly a waste product from the blood and as such incidentally serves to keep the contents of the intestine in a more or less soft condition, thus preventing extreme constipation.
The Liver a Storehouse.--Perhaps the most important function of the liver is the formation within it of a material called _glycogen_, or animal starch. The liver is supplied by blood from two sources. The greater amount of blood received by the liver comes directly from the walls of the stomach and intestine to this organ. It normally contains about one fifth of all the blood in the body. This blood is very rich in food materials, and from it the cells of the liver take out sugars to form glycogen.[43] Glycogen is stored in the liver until such a time as a food is needed that can be quickly oxidized; then it is changed to sugar and carried off by the blood to the tissue which requires it, and there used for this purpose. Glycogen is also stored in the muscles, where it is oxidized to release energy when the muscles are exercised.
Footnote 43: It is known that glycogen _may_ be formed in the body from protein, and possibly from fatty foods.
The Absorption of Digested Food into the Blood.--The object of digestion is to change foods from an insoluble to a soluble form. This has been seen in the study of the action of the various digestive fluids in the body, each of which is seen to aid in dissolving solid foods, changing them to a fluid, and, in case of the bile, actually assisting them to pass through the wall of the intestine. A small amount of digested food may be absorbed by the blood in the blood vessels of the walls of the stomach. Most of the absorption, however, takes place through the walls of the small intestine.
Structure of the Small Intestine.--The small intestine in man is a slender tube nearly twenty feet in length and about one inch in diameter. If the chief function of the small intestine is that of absorption, we must look for adaptations which increase the absorbing surface of the tube. This is gained in part by the inner surface of the tube being thrown into transverse folds which not only retard the rapidity with which food passes down the intestine, but also give more absorbing surface. But far more important for absorption are millions of little projections which cover the inner surface of the small intestine.
The Villi.--So numerous are these projections that the whole surface presents a velvety appearance. Collectively, these structures are called the _villi_ (singular _villus_). They form the chief organs of absorption in the intestine, several thousand being distributed over every square inch of surface. By means of the folds and villi the small intestine is estimated to have an absorbing surface equal to twice that of the surface of the body. Between the villi are found the openings of the _intestinal glands_.
Internal Structure of a Villus.--The internal structure of a villus is best seen in a longitudinal section. We find the outer wall made up of a thin layer of cells, the _epithelial_ layer. It is the duty of these cells to absorb the semifluid food from within the intestine. Underneath these cells lies a network of very tiny blood vessels, while inside of these, occupying the core of the villus, are found spaces which, because of their white appearance after absorption of fats, have been called _lacteals_. (See figure, page 307.)
[Illustration: Diagram to show how the nutrients reach the blood.]
Absorption of Foods.--Let us now attempt to find out exactly how foods are passed from the intestines into the blood. Food substances in solution may be soaked up as a sponge would take up water, or they may pass by osmosis into the cells lining the villus. These cells break down the peptones into a substance that will pass into and become part of the blood. Once within the villus, the sugars and digested proteins pass through tiny blood vessels into the larger vessels comprising the _portal circulation_. These pass through the liver, where, as we have seen, sugar is taken from the blood and stored as glycogen. From the liver, the food within the blood is sent to the heart, from there is pumped to the lungs, from there returns to the heart, and is pumped to the tissues of the body. A large amount of water and some salts are also absorbed through the walls of the stomach and intestine as the food passes on its course. The fats in the form of soaps and fatty acids pass into the space in the center of the villus. Later they are changed into fats again, probably in certain groups of gland cells known as _mesenteric_ glands, and eventually reach the blood by way of the thoracic duct without passing through the liver.
Large Intestine.--The large intestine has somewhat the same structure as the small intestine, except that it lacks the villi and has a greater diameter. Considerable absorption, however, takes place through its walls as the mass of food and refuse material is slowly pushed along by the muscles within its walls.
Vermiform Appendix.--At the point where the small intestine widens to form the large intestine, a baglike pouch is formed. From one side of this pouch is given off a small tube about four inches long, closed at the lower end.
This tube, the rudiment of what is an important part of the food tube in the lower vertebrates, is called the _vermiform appendix_. It has come to have unpleasant notoriety in late years, as the site of serious inflammation.
Constipation.--In the large intestine live millions of bacteria, some of which make and give off poisonous substances known as toxins. These substances are easily absorbed through the walls of the large intestine, and, when they pass into the blood, cause headaches or sometimes serious trouble. Hence it follows that the lower bowel should be emptied of this matter as frequently as possible, at least once a day. Constipation is one of the most serious evils the American people have to deal with, and it is largely brought about by the artificial life which we lead, with its lack of exercise, fresh air, and sleep. Fruit with meals, especially at breakfast, plenty of water between meals and before breakfast, exercise, particularly of the abdominal muscles, and regular habits will all help to correct this evil.
Hygienic Habits of Eating; the Causes and Prevention of Dyspepsia.--From the contents of the foregoing chapter it is evident that the object of the process of digestion is to break up solid food so that it may be absorbed to form part of the blood. Any habits we may form of thoroughly chewing our food will evidently aid in this process. Undoubtedly much of the distress known as dyspepsia is due to too hasty meals with consequent lack of proper mastication of food. The message of Mr. Horace Fletcher in bringing before us the need of proper mastication of food and the attendant evils of overeating is one which we cannot afford to ignore. It is a good rule to go away from the table feeling a little hungry. Eating too much overtaxes the digestive organs and prevents their working to the best advantage. Still another cause of dyspepsia is eating when in a _fatigued_ condition. It is always a good plan to rest a short time before eating, especially after any hard manual work. We have seen how great a part unpleasant emotions play in preventing peristaltic movements of the food tube. Conversely, pleasant conversation, laughter, and fun will help you to digest your meal. Eating between meals is condemned by physicians because it calls the blood to the digestive organs at a time when it should be more active in other parts of the body.
Effect of Alcohol on Digestion.--It is a well-known fact that alcohol extracts water from tissues with which it is in contact. This fact works much harm to the interior surface of the food tube, especially the walls of the stomach, which in the case of a hard drinker are likely to become irritated and much toughened. In very small amounts alcohol stimulates the secretion of the salivary and gastric glands, and thus appears to aid in digestion.
The following results of experiments on dogs, published in the _American Journal of Physiology_, Vol. I, Professor Chittenden of Yale University gives as "strictly comparable," because "they were carried out in succession on the same day." They show that alcohol retards rather than aids in digestion:--
========================================================================== NUMBER OF EXPERIMENT 1/16 LB. MEAT WITH WATER 1/16 LB. MEAT WITH DILUTE ALCOHOL ------------------------+------------------------+------------------------ XVII [alpha] 9:15 A.M. Digested in 3 hours XVII [beta] 3:00 P.M. Digested in 3:15 hours XVIII [alpha] 8:30 A.M. Digested in 2:30 hours XVIII [beta] 2:10 P.M. Digested in 3:00 hours XIX [alpha] 9:00 A.M. Digested in 2:30 hours XIX [beta] 2:30 P.M. Digested in 3:00 hours XX [alpha] 9:15 A.M. Digested in 2:45 hours XX [beta] 2:30 P.M. Digested in 2:15 hours VI [alpha] 9:15 A.M. Digested in 3:45 hours VI [beta] 1:00 P.M. Digested in 3:15 hours ------------------------+------------------------+------------------------ Average 2:42 hours 3:09 hours ------------------------+------------------------+------------------------
As a result of his experiments, Professor Chittenden remarks: "We believe that the results obtained justify the conclusion that gastric digestion as a whole is not materially modified by the introduction of alcoholic fluids with the food. In other words, the unquestionable acceleration of gastric secretion which follows the ingestion of alcoholic beverages is, as a rule, counterbalanced by the inhibitory effect of the alcoholic fluids upon the chemical process of gastric digestion, with perhaps at times a tendency towards preponderance of inhibitory action." Others have come to the same or stronger conclusions as to the undesirable action of alcohol on digestion, as a result of their own experiments.
Effect of Alcohol on the Liver.--The effect of heavy drinking upon the liver is graphically shown in the following table prepared by the Scientific Temperance Federation of Boston, Mass.:--
[Illustration: Proportion of deaths from disease in a certain area due to alcohol. The black area shows deaths due to alcohol.[44]]
Footnote 44: Does not include deaths from general alcoholic paralysis or other organic diseases due to alcohol. Liver cirrhosis due to alcohol conservatively estimated at 75 per cent of total cases.
"Alcoholic indulgence stands almost if not altogether in the front rank of the enemies to be combated in the battle for health."--PROFESSOR WILLIAM T. SEDGWICK.
XXI. THE BLOOD AND ITS CIRCULATION
_Problems.--To discover the composition and uses of the different parts of the blood._ _To find out the means by which the blood is circulated about the body._
LABORATORY SUGGESTIONS
_Demonstration._--Structure of blood, fresh frog's blood and human blood. Drawings.
_Demonstration._--Clotting of blood.
_Demonstration._--Use of models to demonstrate that the heart is a force pump.
_Demonstration._--Capillary circulation in web of frog's foot or tadpole's tail. Drawing.
_Home or laboratory exercise._--On relation of exercise on rate of heart beat.
Function of the Blood.--The chief function of the digestive tract is to change foods to such form that they can be absorbed through the walls of the food tube and become part of the blood.[45]
Footnote 45: This change is due to the action of certain enzymes upon the nutrients in various foods. But we also find that peptones are changed back again to proteins when once in the blood. This appears to be due to the _reversible_ action of the enzymes acting upon them. (See page 307.)
If we examine under the microscope a drop of blood taken from the frog or man, we find it made up of a fluid called _plasma_ and two kinds of bodies, the so-called _red corpuscles_ and _colorless corpuscles_, floating in this plasma.
Composition of Plasma.--The plasma of blood is found to be largely (about 90 per cent) water. It also contains a considerable amount of protein, some sugar, fat, and mineral material. It is, then, the medium which holds the fluid food that has been absorbed from within the intestine. This food is pumped to the body cells where, as work is performed, oxidation takes place and heat is given off as a form of energy. The almost constant temperature of the body is also due to the blood, which brings to the surface of the body much of the heat given off by oxidation of food in the muscles and other tissues. When the blood returns from the tissues where the food is oxidized, the plasma brings back with it to the lungs part of the carbon dioxide liberated where oxidation has taken place. Some waste products, to be spoken of later, are also found in the plasma.
[Illustration: Human blood as seen under the high power of the compound microscope; at the extreme right is a colorless corpuscle.]
The Red Blood Corpuscle; its Structure and Functions.--The red corpuscle in the blood of the frog is a true cell of disklike form, containing a nucleus. The red corpuscle of man is made in the red marrow of bones and in its young stages has a nucleus. In its adult form, however, it lacks a nucleus. Its form is that of a biconcave disk. So small and so numerous are these corpuscles that over five million are found in a cubic centimeter of normal blood. They make up almost one half the total volume of the blood.
The color, which is found to be a dirty yellow when separate corpuscles are viewed under the microscope, is due to a protein material called _haemoglobin_. Haemoglobin contains a large amount of iron. It has the power of uniting very readily with oxygen whenever that gas is abundant, and, after having absorbed it, of giving it up to the surrounding media, when oxygen is there present in smaller amounts than in the corpuscle. This function of carrying oxygen is the most important function of the red corpuscle, although the red corpuscle also removes part of the carbon dioxide from the tissues on their return to the lungs. The taking up of oxygen is accompanied by a change in color of the mass of corpuscles from a dull red to a bright scarlet.
Clotting of Blood.--If fresh beef blood is allowed to stand overnight, it will be found to have separated into two parts, a dark red, almost solid _clot_ and a thin, straw-colored liquid called _serum_. Serum is found to be made up of about 90 per cent water, 8 per cent protein, 1 per cent other organic foods, and 1 per cent mineral substances. In these respects it very closely resembles the fluid food that is absorbed from the intestines.
If another jar of fresh beef blood is poured into a pan and briskly whipped with a bundle of little rods (or with an egg beater), a stringy substance will be found to stick to the rods. This, if washed carefully, is seen to be almost colorless. Tested with nitric acid and ammonia, it is found to contain a protein substance which is called _fibrin_.
Blood plasma, then, is made up of a fluid portion of serum, and fibrin, which, although in a fluid state in the blood vessels within the body, coagulates when blood is removed from the blood vessels. This coagulation aids in making a blood clot. A clot is simply a mass of fibrin threads with a large number of corpuscles tangled within. The clotting of blood is of great physiological importance, for otherwise we might bleed to death even from a small wound.
Blood Plates.--In blood within the circulatory system of the body, the fibrin is held in a fluid state called _fibrinogen_. An enzyme, acting upon this fibrinogen, the soluble protein in the blood, causes it to change to an insoluble form, the fibrin of the clot. This change seems to be due to the action of minute bodies in the blood known as _blood plates_. Under abnormal conditions these blood plates break down, releasing some substances which eventually cause this enzyme to do its work.
[Illustration: A small artery (_A_) breaking up into capillaries (_c_) which unite to form a vein (_V_). Note at (_P_) several colorless corpuscles, which are fighting bacteria at that point.]
The Colorless Corpuscle; Structure and Functions.--A colorless corpuscle is a cell irregular in outline, the shape of which is constantly changing.