Popular Science Monthly/Volume 17/September 1880/How Animals Digest
|HOW ANIMALS DIGEST.|
IN reception of food, animals have been compared to plants turned outside in. The plant absorbs nourishment by pores in the foliage and rootlets. Higher animals absorb food by similar closed tubes which line a cavity of the body. This interior cavity, the food-tract or alimentary canal, is the most important and the most nearly universal organ of the animal structure. Its purpose is threefold—that of a reservoir, as animals can not always procure their proper food and can not, like plants, be ever eating; a liquefier, as all food, both for plants and animals, must be in the fluid state; and, thirdly, a chemical laboratory, as nourishment must be like the body in composition. Plants have no need of such an organ, because the food is always at hand in proper condition. Consequently, food is not in the animal body proper when in the stomach. It is within the body, not of the body. It is only dead matter, under the control of the organic forces of the body, and preparing to become a substantial part of the living organism.
The several processes of animal digestion, mechanical, physical, and chemical, are simultaneously performed in varying degree throughout the whole length of the digestive tract. For our present purpose, however, it is more practicable to describe them separately. But that the division is arbitrary should constantly be kept in mind. The main part of mechanical digestion has already been described in the article on "How Animals eat."
Deglutition.—In animals which have the stomach some distance from the mouth or oral aperture, the swallowing of food is a distinct act, requiring special organs. The food must be forced through the connecting tube, known as the œsophagus or gullet. We do not find such in the lowest animals. The whole of nutrition is a single process in the tape-worm; with the amoeba, grasping of food is not distinct from digestion; while in the anemone and jelly-fish the mouth opens directly into the stomach cavity.
If the stomach were always beneath the mouth, as in man and birds, food might with some difficulty reach the former by gravity. Birds do help the descent of food by jerking the head, and most birds in drinking lift the head each time the beak is filled. But animals must be able to swallow in spite of gravity, as commonly in eating the stomach is higher than the mouth. This is possible even in man, for the juggler drinks when standing on his head.
Deglutition is accomplished by a peculiar and beautiful involuntary action of the gullet. The walls of this tube are composed of two muscular layers, longitudinal and circular, which act in accord. Immediately in front of the bolus of food the walls are relaxed, while behind and around it the walls contract, thus urging the matter forward; and, "as it travels, the wave of contraction travels with it." This motion of the gullet is well shown in the neck of a horse when drinking. It is a similar action which propels the food through the entire length of the digestive canal—termed in the intestines peristaltic or vermicular motion. The mill-like action of the gizzard and the churning motion of the stomach are only phases of the same thing. By a reversed action of the gullet, the cud of an ox is thrown from the stomach back to the mouth.
To place the bolus of food within reach of the muscles of the gullet, there is in the highest animals a most complex arrangement of parts in the pharynx or back of the mouth. In mammals, the pharynx is a funnel-shaped cavity having seven openings. Here the gullet crosses the air-pipe; and, to keep food and drink from taking the wrong road, there is an effective system of valves. The parts act spasmodically whenever they are irritated by the pressure of solids and liquids.
Saliva is found in nearly all animals. Its universal office is to lubricate the food and so help it to glide easily along the pharynx and
gullet. This kind of saliva is a glairy mucus, and is the only kind in animals which do not chew the food—for example, birds, reptiles, and fishes. As an aid to digestion, the saliva of mammals will be considered later.
The most astonishing feats in swallowing are performed by the snakes. The boa can certainly swallow a goat or deer. Our common little snakes, the size of a finger, can swallow a large frog, a performance sufficiently remarkable. The process is very slow and tedious, and one would suppose painful. The boa first kills its prey by crushing it in its tightening coils, which break down the ribs and limbs and reduce the victim to a shapeless mass. By this horrible proceeding the carcass is gotten into condition to be more easily swallowed. After coating it with mucus, the boa begins the difficult operation of forcing the huge mouthful down its throat. But how shall the act be accomplished with no limbs to assist? As the under jaw divides in front, and articulates with the skull by the intervention of extra movable bones, the mouth and throat can stretch enormously. The sharp, conical teeth are recurved, acting like the barb of an arrow to hold whatever position is gained. Each side of the jaw is pushed forward in turn and gains a new and further hold on the carcass, which by successive slight movements is slowly pulled head first down the gullet. The common striped snake seizes a toad or frog, however he can catch him, usually by one or both hind legs, and immediately proceeds to "take him in," despite all protests and struggles.
As the opposite of the enormous throat of the snake, the bulky whalebone whale has the smallest throat, proportionate to its size, of any animal just large enough to admit the tiny creatures which are its food. We see in this a fine example of Nature's economy.
The alligator has a curious way of preventing the admission of water when swallowing prey. Seizing a fish or other small creature, the reptile rises to the surface of the water and flings it into the air; then, before it reaches the water, catches it and gulps it down. If the prey is too large to handle in this manner, it is carried to the shore to be devoured.
Ingluviation.—Many animals can not procure their particular food at all times. Such either can endure fasting, like members of the cat tribe, or have a special reservoir, as shown in the crop of a fowl. This crop is only a dilatation of the gullet. In the cormorant, the whole gullet is very capacious, for the purpose of storing fish; on account of which habit the bird has become a type of voracity. The pigeon has its crop divided into two—perhaps to give a better form for flight. The pelican has a bag beneath the lower jaw. Many small animals, insects especially, have crops. Similar in purpose is the first stomach, or paunch, of a cud-chewer. Birds which eat fruit, insects, or other food readily procured, and of a character which needs no delay in digestion, have usually no crop, or but a rudimentary one.
While the whole digestive tract serves the purpose of a reservoir, the special reservoirs have indeed a digestive function, serving to delay the food, that it may be acted upon for a sufficient time by the chemical fluids. Thus the crop of a bird secretes a fluid which softens and prepares the hard grain for subsequent trituration and digestion.
Organs of Chemical Digestion.—As the organ of digestion proper is the one most nearly universal, it consequently affords the Fig. 3.—Tænia solium, or Solitary Worm: a, head, or scolex; b, tape formed of many individuals, the last of which, completely sexual, separate under the name of proglottides, and represent the adultand complete anima. Each solitary worm is a colony. finest example of specialization and development. From the improvised cavity of the amœba, there is a steady progress by minute steps to the complex apparatus of the mammals. Digestion is not more perfect, however, in the latter than in the former. The simple nutritive act of the amoeba is as perfect for itself as the differentiated process of the highest animals is for them. In the lowest animals, the function is single, and so simple that no special organ is necessary. As we rise in the animal scale, the function is divided into secondary functions, which require for their performance a corresponding number of special organs. Indeed, the complex functions of prehension, mastication, digestion, and circulation are only subdivisions of nutrition which begins in the lowest life as a single act. The present purpose, however, is not to trace the evolution of specialization of the digestive function further than to illustrate its general principles and methods, and present some of its peculiar and interesting features.
The tape-worm has no digestive organs whatever, having no use for them. A robber subsisting on the labors of its victim, it takes food in the same manner as a plant, by absorption from the outside. This is also the case with many lower protozoa.
The digestion of the amœba is only one remove higher than that of the tape-worm—with no permanent organs, but extemporizing a stomach from the skin as required. A step higher still we find the hydra, with a permanent body cavity serving the purpose of a stomach. But it is not distinctively a stomach, as it is the common organ of all the other vegetative functions. A single opening serves both to receive the food and expel the waste matter. Within even this narrow limit of structure, we find a host of low animals which exhibit a great variety of forms; so that from the hydra to the ctenophore is a progressive series showing a gradual specialization of this common
|Pond-Amœba digesting its Food.||Amœba eating.|
organ. In the higher part of the series, as for example the sea-anemone, there is a digestive cavity somewhat separated from the body cavity, though still connecting; and all the excretions have to find their way out through the oral aperture.
In the compound hydrozoans, produced by budding and division, such as sertularia and the so-called corals, the body cavity is continuous through the whole community. Hence each individual (though
it is scarcely correct to regard it as such) has its stomach connected with the stomachs of all the others. Whatever food one digests serves to nourish the whole colony. They are absolute communists.
At this point should be presented the fact that in all animals the lining or secreting membrane of the food-canal is essentially but a continuation of the skin. That such is true of the amœba is evident, for what was the outside of the body-mass becomes when food is enveloped the lining of the new cavity. The cup-shaped body of the hydra can be turned inside out without interference with the business of digestion. The skin in these cases must have a chemical digestive power,
as the food taken in mass, and frequently living substance, or even whole animals, is dissolved without trituration or mechanical aid.Fig. 7.—Digestive System of a Beetle (Carabus auratus): a, œsophagus; b, crop; c, gizzard; d, chylific stomach; e, Malpighian tubes; f, intestine; g, cloaca; h, supposed renal vessels. The human skin has powers of absorption, and in some slight degree a person may be fed through it. The continuation of the skin which lines the digestive canal is supplied with new powers of secretion; so that, instead of producing perspirations, oils, etc., it manufactures chemicals for changing food. Hence the principle of digestion and the character of the organs are fundamentally the same in all animals, the lowest with the highest. Only in the amoeba, hydra, etc., digestion is accomplished with the least possible expenditure.
A true stomach must be wholly devoted to the elaboration of food, leaving other functions to other organs. This requires that it be wholly shut off from other cavities of the body; and it were better to have two openings, one for reception of food, the other an outlet for waste matter, in order to give the food a single direction and prevent the mingling of digested and undigested matter. This perfect stomach is realized so gradually, or by such slight degrees, in a large number of lower animals, that it is not easy to say positively which animal has the honor of its first possession. To the little sea-urchin, which is the first possessor of true teeth, is generally given the credit. The ctenophore, a lower animal, has indeed two or more apertures to its food-cavity, but the mouth is still the main excretory orifice, and this cavity freely communicates with a system of body canals. The starfish, on the contrary, has the stomach distinct from the other cavities of the body, with, however, in most cases only a single opening.
In the sea-urchin we also find an intestine and indications of the several parts which are so distinct in higher animals. The sea-cucumber,
a near relative of the former but with a better digestive canal, is almost as highly favored regarding its stomach as the low amoeba. If its stomach becomes troublesome from indigestion or other cause, it simply ejects it through the mouth, along with its other internal organs. Then it quietly awaits the growth of a new set—certainly a very happy and efficient method. Many human dyspeptics would rejoice in the same power. This animal is said to reject its viscera when it is injured or alarmed. This is interesting, as showing in the low animals that which is well known in the highest, the immediate effect of fear and pain upon the internal organs, or the close dependence of the nutritive organs upon the nervous system.
Among articulates and mollusks we find a great diversity in the character of the digestive canal. Its main divisions are always shown more or less distinctly. But in many articulates, as some worms, myriapods, larvae of insects, and crustaceans, the tube is quite straight; while in others it is highly convoluted. Biting insects have all the parts ever found in the food-tract, namely, pharynx, gullet, crop, gizzard, stomach, small and large intestines. In some members of the spider family, the short and straight food-canal sends off branches into the limbs and other members. The absurdity of a creature carrying its stomach in its legs!
Many snails have crop and gizzard, as also has the nautilus. In snails the intestine passes through the liver, and in clams through the heart.Fig. 9.—Diagram of the Digestive System of a Mammal: g, gullet; s, stomach; sm, small intestine; lm, large intestine; r, rectum, terminating in the aperture of the anus. Many butterflies take no food, and the digestive organs are entirely absent. In this case the eating and storing of nutriment was performed in the earlier larval state with excellent organs. But the male notommata, one of the rotifers, never has digestive organs; it lives its brief life upon the nourishment of the egg from which it was derived.
Fishes have a short and comparatively simple alimentary tube with generally a wide gullet, and seem commonly to disgorge indigestible substances. Reptiles usually have, the parts more strongly marked. Tadpoles have a very long and greatly convoluted canal, but the vegetable-eating turtles have the longest. Crocodiles have a powerful gizzard, like birds, and are said to swallow stones to assist the trituration of food. They approach birds also in possessing a mesentery, a membrane which supports the food-tract and fastens it to the walls of the body. In all lower animals the canal lies loosely in the body cavity. The food-tract of birds varies in length and character according to the kind of food. The crop and gizzard have already been described, the latter in a former article.
In mammals the great body cavity is divided into two chambers, thorax and abdomen, by a transverse partition called the diaphragm. The gullet passes through and the stomach lies just beneath this membrane. The parts of the food-tract are always clearly marked, and the stomach is frequently divided. In all animals digestion is more prolonged in proportion as the food is unlike animal substance. With carnivorous mammals the process is simple. The whole length of the food-tract in members of the cat tribe is only about three times the length of the body. Man employs a mixed diet, and has the canal six times the length of the body. No food is less like flesh than herbage, consequently we find with such food the most difficult digestion. In ruminants the canal is over twenty times the length of the body; and the stomach is divided into four chambers, to delay the food and complete the mechanical process. The first two, however, are more properly expansions of the gullet. The first chamber, called the paunch or rumen, stores and moistens the half-chewed food. This division in the camel has a portion lined with cells for storing water. The second chamber, known as the reticulum, or honey-comb stomach, receives this raw material, rolls and presses it into separate balls, which are sent up to the mouth for more perfect mastication. When this is complete, the now semi-fluid or pasty bolus, being unable to distend the aperture to the first and second chambers, flows into the third chamber. This has its lining greatly folded in order to detain coarse material, whence it is called manyplies or psalterium. The fourth chamber,
abomasum, is the stomach proper, as here alone is the food subjected to the gastric juice.
On account of defective mastication, the whales have at least three divisions of the stomach; many mammals have two divisions; and the toothless ant-eater has a gizzard.
Digestive Fluids.—After considerable investigation, the precise action of the several fluids which accomplish the chemical change of food is yet unknown. Indeed, their general function is still a matter in discussion. Naturally, then, our knowledge of the functions of the accessory digestive organs in the lower animals is limited. That digestion, however, in all animals is the result of chemical action under the influence of vital force seems assured.
Of the several fluids or chemical agents prepared within the laboratory of the digestive apparatus, the most important and indispensable is the gastric juice, the presence of which determines the location in the food-tract of the stomach proper. This is because the fluid is produced by the lining of the stomach, and not by a distinct organ. That even the microscopic animals have some digestive fluid, like gastric juice, is regarded as proved by the fact, already noticed, that solid food is dissolved by them without mechanical aid. This fluid is well shown in the radiated animals. Its active principle is a ferment called pepsin, which acts only in the presence of an acid. The acidity of the fluid is given by free hydrochloric acid. Gastric juice dissolves only nitrogenous substances, as meat, albumen, and gelatine, having little or no effect on oil or starch.
Next to the gastric juice in importance, if we may judge by its early appearance in the animal kingdom, is the bile. This alkaline fluid is found in all animals having a distinct digestive cavity. The earliest biliary organs are minute cells upon the stomach-lining, as in the anemone. A higher form is found in the small tubes surrounding
the intestines of the insect, from which there are slow gradations to the superior liver of the higher mollusks and fishes. In the articulates, mollusks, and all higher animals, the bile is poured into the intestine and so separated from the gastric juice. The action of the bile is not fully known; but it appears to dissolve fats slightly, and helps to subdivide them into minute particles which are "diffused through the liquid like atoms of butter in milk." It probably aids also in the process of absorption.
The pancreatic juice, another alkaline fluid, is found below the vertebrates only in the higher mollusks. As a gland the pancreas is rudimentary in the cephalopods, but appears better developed in the fishes, and proportionally largest in birds. The function of this fluid is a general one, as it acts on nearly all aliments, and seems to be the principal means of digesting oils and starch, or carbonaceous foods. It is poured into the small intestine near the stomach.
By the mucous lining of the intestines there is produced an alkaline fluid or fluids which are supplementary to the former. Thus we find digestive fluids secreted through the whole length of the food-canal.
Saliva is present in most animals as a lubricant for the food. But in those animals which chew the food there occurs another kind of saliva, a limpid fluid which aids mastication by softening the food. In mammals this has also a chemical power, changing starch to sugar. As the latter substance is heat-producing, this chemical energy is lacking in the cold-blooded vertebrates. In birds this saliva is replaced by the abundant pancreatic juice. It is most abundant in herbivores, as might be supposed from the fact that starch is a vegetable product.
Absorption.—The dissolved and chemically altered food is yet to be taken into the body, and carried wherever needed, either to supply deficiency or produce growth. At present we have to do only with the first process. In some degree the food is absorbed, as fast as digested, by blood-vessels through the whole alimentary tract. This is true particularly of the stomach. But in vertebrates absorption is
chiefly by minute tubes, called lacteals, which line the intestines. After passing through certain glands, these tubes unite to form a single tube known as the thoracic duct, which pours its contents into the veins in the neck.
In the higher invertebrates the blood-vessels take up all the nutriment directly from the digestive canal. In the lower, the food as fast as digested passes directly through the walls of the canal into the tissues; while in the lowest animals, where the digestive cavity communicates with the body cavity, the food freely bathes all parts of the structure. Simpler still, in the case of the tape-worm absorption is all the creature has to do.
Whether the process of absorption is wholly physical or partly vital is disputed. But some time during the process, or immediately afterward, the food is changed from merely dead substance to vitalized organized matter, and it is now ready to form part of the living tissues.