The Food Tube
In the lowest unicellular forms of animal life, the osmotic process of taking in food substances is performed by the outside of the body, somewhat after the fashion of plants, as most simply demonstrated by Amoeba.
Among sponges, which take the first step in the great adventure of cell associations, the method of intake is hardly different, although there is a a prophecy of an internal digestive tube in the ciliated passage-ways that honeycomb the loosely connected sponge mass, through which the foodladen water is made to stream.
Hydras, corals, and sea-anemones, as well as all other typical coelenterates, have a digestive sac open only at one end, and little else. This single external opening serves as both mouth and anus. In these pioneer animals everything is sacrified to securing a suitable place for the bestowal of food. The very shape of the body is determined by the food sac, for the whole animal is simply an animated food bag, decorated around the intake opening with a fringe of subservient tentacles. The importance of the food cavity is thus clearly emphasized by its early establishment before most other structural refinements peculiar to the animal organism.
Even in echinoderms, although an anus is nominally present it plays only an occasional role, since these devastating, devouring creatures, of which starfishes and sea-urchins are typical examples, dispose so effectually of the food entering their maw, that there is very little waste left over for expulsion at the exit. As a matter of fact in the case of the starfish, most of the food waste is not even taken into the mouth. Instead the stomach is everted from the mouth in feeding and enwraps the food or prey so completely that the indigestible parts are left behind when the stomach is withdrawn, leaving no residue to be passed out at the anus.
Worms and caterpillars may well be described as perambulating digestive tubes, with the important mouth end pointed toward a food-containing world. Directive sense organs cluster around this exploratory end of the food tube, informing it where to go.
A vertebrate in reality is a double tube. The outer tube is the protective body wall, and the inner tube, the digestive canal. Between the two tubes is the body cavity, which makes possible within a limited space the storage of a digestive canal much longer and more efficient than the exterior of the animal would lead one to suspect. Thus, the knapsack for carrying the rations is bestowed within the body instead of being carried outside.
Increase in Digestive Surface
So long as the bulk of an animal’s body remains small a straight digestive tube has an adequate internal surface to meet all alimentary demands. It is mathematically demonstrable, however, that while the surfaces of two homologous solids are to each other as the squares, the masses are to each other as the cubes of their homologous dimensions. This means that the bulk of a growing animal increases more rapidly than its surface, with the inevitable result that a straight unmodified digestive tube becomes inadequate to take care of the accompanying mass. This is particularly true in the case of herbivores, whose food is less concentrated than that of carnivores, and who consequently need digestive machinery adequate for handling a larger quantity of food in a given time.
There are four general ways in which this need for increase of digestive surface has been met in various animals, namely, (a) by increase in diameter; (b) by increase in length; (c) by internal folds and elevations of various kinds; and (d) by the addition of supplementary diverticula.
Increase in Diameter
This method is not extensively employed, because of the limitations of space in the body cavity. If the inner tube increases in diameter the outer tube of the body wall must also enlarge, which tends to defeat the object to be gained. Certain regions of nearly every digestive tube, such as the stomach and large intestine, are frequently, nevertheless, of greater diameter than the remainder of the tube.
Increase in Length
Increase in length is a universal device among vertebrates for adding to the available digestive surface, since the body cavity furnishes possible space for stowing away coils and loops of the tube. The body cavity not only makes a place for an intestine longer than the body itself, but it also frees the intestinal tube from the muscular control of surrounding tissues, permitting it freedom to exercise peristaltic movements of its own.
The characteristic swollen shape of a tadpole, resembling an animated head with a tail attached, is due to the enormously lengthened digestive tube which is coiled about many times, packing the body cavity full. Just before metamorphosis, when the tadpole gut is adapted for plant food, it may measure eight to ten centimeters in length, whereas after metamorphosis, when the young frog switches over to insect food thus requiring less digestive surface, the tube shortens to three or four centimeters in length, although the body itself is now considerably longer than before (Fig. 210).
In man the entire digestive tube is between twenty-five and thirty feet in length, although the entrance and exit are only about two feet apart.
Increases both in diameter and length of the digestive tube make demands that soon encroach upon limits of possible space within the body cavity. Internal folds within the food tube itself avoid this difficulty by adding to the expanse of surface to which the food is exposed without adding to the external size of the tube.
A longitudinal fold extending into the cavity of the tube is termed a typhlosole (Fig. 211). Such an arrangement is present in the cyclostomes. In dipnoans, as well as elasmobranchs and ganoid fishes, the intestinal part of the food tube is supplied with a spiral valve (Fig. 212), a typhlosole so much longer than the tube in which it is placed that it must coil around like a spiral stairway, with one edge attached while the other is free.
Certain invading transverse folds, called plicae circulares (Fig. 213), give a washboard effect to the inner surface of the anterior part of the human intestine, while countless tiny elevations, or villi, projecting like the nap of velvet from the inner surface of the small intestine, particularly in the higher vertebrates, produce, in a minimum of space, an enormous increase of absorbing area for contact with passing food.
Side alleys, or diverticula, from the main tube occur in many instances. These are particularly abundant in fishes at the junction of the stomach with the small intestine, where they are called pyloric caeca (Fig. 214). They vary in number from one in the ganoid Polypterus and the sand-lance Arnmodytes to over 200 in the mackerel Scomber.
Other diverticula, called colic caeca, are found at the junction of the small and large intestines in amniotes. The colic caecum of a turtle is only a slight enlargement (Fig. 215), but in rabbits and some rodents it may become an enormously enlarged tube with an internal capacity nearly equal to that of the rest of the digestive canal to which it is attached.
In man the colic caecum, with its troublesome shriveled prolongation, the processus vermiformis, or “vermiform appendix” (Fig. 216), has outlived its usefulness and bears an unsavory reputation. Birds have typically two colic caeca (Fig. 217).
The large intestine of man, as well as of several other mammals, is pushed out into a series of baywindow-like enlargements (Fig. 218), which are diverticula of a sort called haustra. Connected with the rectal region throughout the vertebrate series are various problematical outpushings, such as the rectal gland of elasmobranchs; the urinary bladder of amphibians; the bursa of Fabricius in birds, and the anal glands of certain mammals, all of which have been made to serve different uses, although not necessarily connected with the process of digestion.
Mouth and anus are not essential during embryonic development when the body secures its nourishment either from the yolk mass or, in the case of mammals, from the maternal blood stream. There comes a time, however, when provision must be made to admit food into the digestive cavity from other sources. This necessity is met by the formation of two ectodermal invaginations, one near each end of the elongated archenteron, which break through to make a continuous open passage-way, the digestive tube.
The anterior ectodermal ingrowth is called the stomodaeum, and the posterior ectodermal part, the proctodaeum, while the endodermal region between them, which was originally the archenteron, is now termed the mesodaeum. The embryonic stomodaeum stakes out the claim for the future mouth region; the proctodaeum locates the anus. The food tube thus consists of three embryonic components, although the landmarks that separate them from each other are obliterated in the adult.
A cross section of the digestive tube within the body cavity shows it to be made up of several concentric layers ol cells (Fig. 219).
The innermost layer, or mucosa, the original embryonic endoderm, is supported by mesodermal connective tissue, the submucosa. I he mucosa is only one cell-layer thick, except in the anterior esophageal region. It performs not only the “root hair” function of absorption, but also gives rise to various digestive glands whose secretions bring about chemical transformation of the food taken in. All the other layers aside from the mucosa are secondary and are subsequently added to this most important absorptive primary lining of the food tube.
The submucosa, next to the mucosa, is largely devoted to supporting a rich network of capillaries and lymphatics which bear away over the body the materials absorbed by the mucosa.
Outside of the submucosa there is a double layer, the muscularis, composed of circular muscles on the inside and longitudinal muscles on the outside. These muscles are involuntary in their action, except for a short distance at either end of the tube in the stomodaeal and proctodaeal regions, where they are under the control of the will. They effect movement of the food through alternate contractions by processes of segmentation and peristalsis. Segmentation churns the contents of the tube back and forth, while peristalsis forwards it.
Protecting the muscular layers on the outside is a sustentative layer of tissue called the serosa, which is continuous with the mesenteries and with the peritoneum that lines the body cavity. In that part of the tube lying outside of the abdominal cavity, no serosa is present.
Regions of the Tube
Since food undergoes progressive modification as it passes through the digestive tube, the tube itself, as would be expected, shows structural adaptations for the performance of these various tasks. Of necessity there has evolved a physiological division of labor, or specialization, which has left its mark on the morphological features that characterize the alimentary tract in different regions. For purposes of description the entire tube may be divided into four zones, or regions, namely, ingressive, progressive, degressive, and egressive.
The ingressive zone is the intake region of prehension and mastication. It involves the lips and mouth with the teeth, tongue, and various other structures contained therein. The progressive zone, embracing the pharynx, esophagus, and stomach, is the region of forwarding the food-intake and passing it through the preliminary stages of modification. The degressive zone, coincident with the small intestine, is not only the most extensive but also in a sense the most important part of all the zones, for here occurs the chemical preparation of the food stuffs, and their ultimate selection and absorption into the blood. Finally, the egressive zone, which is confined to the large intestine, is the region for the expulsion of the unusable residue that cannot be diverted into the blood and applied to the uses of the body. These regions are shown diagrammatically in silhouette for fishes, amphibians, birds and mammals, in Figure 220. With this introduction we may now proceed upon an imaginary tour of inspection through the entire alimentary tract, with our eyes open for the anatomical scenery along the way.