STOMACH.
The distribution of the vagus nerve finds its explanation in this derivation of the stomach. The primitive foregut is formed by the passage between the branchial cavity and the midgut, and is within the area supplied by the vagus. Hence when the stomach develops from the foregut, as a specialized segment of the same, it is supplied by vagus branches. The vertebrate stomach varies greatly in size and shape.
The type-form is presented by a longitudinal spindle-shaped dilatation of the foregut, which retains its fœtal vertical position in the long axis of the body. An example of this form, which is encountered among fishes and amphibia, is presented by the alimentary tube of Proteus anguineus and Necturus maculatus (Figs. 50 and 51). Since this condition is common to all vertebrates in the earliest fœtal period it can be designated as the fœtal or primitive stomach form. All others appear as secondary derivatives from this typical early condition.
The influences which bring about such derivations and modifications may be enumerated as follows:
1. The habitual amount of food required by the animal.
2. The volume and digestible character of the food.
Fig. 52.—Alimentary canal of Coluber natrix. (Nuhn.)
3. The size and shape of the abdominal cavity in which the stomach is contained.
4. Structural modifications designed to increase the action of the gastric juice on the food contained in the stomach.
5. The assumption, on part of the stomach, of functions which are usually relegated to other organs.
Most of the individual stomach forms encountered among vertebrates owe their production to several of these influences acting in conjunction.
We may group the main types as follows:
1. Stomach Forms Depending on the Influence exerted by the Habitual Amount of Food required by the Animal.—The greater the activity of tissue changes is, the greater will be the amount of food required and the more pronounced will be the gastric dilatation of the alimentary canal. Hence in the higher vertebrates generally the stomach appears as a large and more sac-like dilatation than in lower forms, such as fishes and amphibia and some reptilia, in which the stomach is usually smaller and fœtal in shape, forming a slight longitudinal dilatation situated in the long axis of the body. An example is seen in the stomach of Coluber natrix (Fig. 52). Frequently this slight dilatation is scarcely differentiated from the œsophagus at the cephalic and from the small intestine at the caudal end. Many batrachians and perennibranchiates possess this form among the amphibia. It is also encountered in the pickerels, the Cyprini, and in Labrus among fishes, and in some saurians and ophidia among reptiles. It constitutes a slight advance in development over the earliest stage represented, as we have seen, by the nearly uniform and undifferentiated alimentary tube of amphioxus and the cyclostomata.
This transition of the fœtal form to the more advanced secondary types of the stomach is marked by the development of two important structural features:
(a) The separation in the interior of the canal of the stomach from the intestine by the appearance of a ring-shaped valve, the pyloric valve. This is produced by an aggregation of the circular muscular fibers of the intestine at this point, and causes a projection of the mucous membrane into the lumen of the canal. It begins to appear in the fishes (pickerel, sturgeon, etc.), is found in most amphibia and is regularly present in the stomach of the higher vertebrates. (Figs. 54 and 55.) A good example of the ring-shaped plate of the pylorus with central circular opening produced by the aggregation of the circular muscular fibers is afforded by the view of the interior of the cormorant’s stomach given in [Fig. 69]. The opposite or œsophageal extremity of the stomach is less well differentiated from the afferent tube of the œsophagus.
There is no aggregation of muscular circular fibers in this situation and no valve. Superficially the external longitudinal muscular fibers of the œsophagus pass continuously and without demarcation into the superficial gastric muscular layer. The separation between œsophagus and stomach is, however, marked on the mucous surface by a well-defined line along which the flat, smooth and glistening œsophageal tesselated epithelium passes into the granular cuboidal epithelium of the gastric mucous membrane. The œsophageo-gastric junction in the adult human subject is shown in Fig. 53.
(b) The pyloric end of the stomach makes an angular bend, while the rest of the organ remains in the original vertical position in the long axis of the body. An example of this condition is presented by the stomach of Scincus ocellatus ([Fig. 56]; cf. also [Fig. 202]).
The purpose of both of these provisions is to retain the gastric contents for a longer time within the stomach. Hence this form is encountered especially in those fishes and amphibians in which the nutritive demands require a more complete digestion of the food taken. This is the case, for example, in Gobius ([Fig. 57]), the plagiostomata ([Fig. 58]), and many saurians. The same transitory stomach form is even found in some mammals, as the seals. [Fig. 59] shows the stomach in Phoca vitulina, the harbor seal. With the further increase in the demand for complete digestion of the food the entire stomach assumes a transverse position to the long axis of the body. This may occur while the stomach still retains its primitive tubular form, as in most chelonians (Fig. 60). In others the change in position occurs after the gastric dilatation has assumed the sac-like form, as in many land-turtles, crocodiles, some batrachians and all higher vertebrates ([Figs. 61] and [62]). This transverse position, at right angles to the long axis of the body, forms the starting point for the derivation of all secondary types of stomach.
| Fig. 59.—Stomach of Phoca vitulina, harbor seal. (Columbia University Museum, No. 600.) | Fig. 60.—Stomach of Pseudemys elegans, pond turtle. (Columbia University Museum, No. 1710.) |
| Fig. 61.—Stomach of Chelydra serpentina, snapping turtle. (Columbia University Museum, No. 1852.) | Fig. 62.—Same in section. |
2. Stomach Forms Depending on the Influence Exerted by the Volume and Digestible Character of the Foods.—Vegetable substances usually have a large volume in proportion to the amount of nutritive material which they contain. Meat, on the other hand, contains considerable nutriment in a comparatively small bulk. Hence carnivora (Fig. 63) usually have a smaller stomach than herbivora (Fig. 64).
| Fig. 63.—Stomach of Lutra vulgaris, otter. (Nuhn.) | Fig. 64.—Stomach of Equus caballus, horse. (Nuhn.) |
3. Stomach Forms Influenced by Size and Shape of the Abdominal Cavity in which they are Contained.—In animals whose bodies are long and slender, as in snakes ([Fig. 52]), most saurians ([Fig. 56]), many tailed batrachians and perennibranchiates ([Figs. 50] and [51]), many teleosts ([Fig. 48]), the stomach is likewise usually long and slender in shape, unless special modifying conditions exist. When on the other hand the body is broad and short, as in Lophius (Fig. 65), Pipa (Fig. 66), and most higher vertebrates, the stomach is also broader and more sac-like.
| Fig. 65.—Stomach of Lophius piscatorius, angler. (Nuhn.) | Fig. 66.—Stomach of Pipa verucosa. (Nuhn.) |
4. Stomach Forms Depending on Structural Modifications Designed to Increase the Action of the Gastric Juice on the Food.—This purpose is accomplished:
(a) By increasing the source of supply of the gastric juice.
(b) By increasing the length of time during which the food remains in the stomach.
| Fig. 67.—Stomach of Castor fiber, beaver. (Nuhn.) | Fig. 68.—Stomach of Manatus americanus, manatee. (Nuhn.) |
Fig. 69.—Stomach of Phalacrocorax dilophus, double-crested cormorant; section. (Columbia University Museum, No. 67/1804.)
(a) The source of supply of the gastric juice is increased by adding to the usual gastric glands of the stomach a special accessory glandular compartment, either placed at the cardia, where the œsophagus enters, as in Myoxus or Castor (Fig. 67) or attached to the body of the stomach to the left of the cardia, as in the manatee (Fig. 68). The first arrangement is similar to the universal position of the glandular stomach of birds (Fig. 69). In birds, however, the glandular proventriculus is the only source of the gastric juice, while in the above-mentioned mammalia (myoxus and beaver) the accessory glandular stomach is merely an addition to the supply derived from the usual gastric glands situated in the body of the organ.
(b) The increase of the length of time during which the food remains in the stomach subject to the action of the gastric juice can be accomplished in one of several ways.
1. The stomach, while it retains its general tubular form increases considerably in length and assumes the shape and structure found in the human large intestine. It is partially subdivided by folds projecting into the interior and separating compartments resembling the colic cells of the human large intestine. The time required for the passage of food through the stomach is thus increased and the action of the gastric juice is prolonged and rendered more intense.
Such modifications of the structure of the stomach are encountered in Semnopithecus among the monkeys and in the kangaroo, among marsupials (Figs. 70 and 71).
2. The same purpose is accomplished by the development of diverticula from the stomach, in which the food is retained and acted on by the gastric juice for longer periods.
| Fig. 70.—Stomach of Halmaturus derbyanus, rock kangaroo. (Columbia University Museum, No. 582.) | Fig. 71.—Stomach of Semnopithecus entellus, entellus monkey. (Columbia University Museum, No. 62/1805.) |
Fig. 72.—Alimentary canal of Anguilla anguilla, eel. (Columbia University Museum, No. 1271.)
The herbivora, omnivora and such carnivora as live on animal food difficult of digestion furnish examples of this type of stomach. The same is also found in most teleosts. In the latter the cæcal gastric pouch lies in the long axis of the body, opposite the entrance of the œsophagus. A marked example of this arrangement is seen in the stomach of the eel, Anguilla anguilla (Fig. 72).
In other forms, and in the mammalia especially, the blind pouch is developed from the portion of the stomach lying to the left of the œsophageal entrance at the cardia, and is hence placed transversely to the long axis of the body.
This difference in the position of the cul-de-sac is explained by the small transverse measure of the body in teleosts, while the greater amount of available space in the abdominal cavity of mammalia permits of the transverse position of the entire stomach and of the development of the diverticulum from its left extremity.
Most mammals have only a single pouch, whose size varies with the digestibility of the food habitually taken. It is greater in herbivora ([Figs. 64] and 73) than in omnivora and carnivora (Figs. 74 and 75). In some of the latter, as Lutra ([Fig. 63]), the cul-de-sac is almost wanting.
In some forms, as the pig, the left extremity of the stomach carries a cæcal appendix with a spiral valve in the interior separating its lumen from the general gastric cavity (Fig. 78). Others have two such cæcal appendices added to the left end of the stomach (Peccary, Fig. 79). These cæcal pouches may arise from the body of the stomach, instead of from the left extremity. An example of this condition is furnished by the American manatee ([Fig. 68]).
Fig. 79.—Stomach of Dicotyles torquatus, peccary. The fundus is a capacious pouch prolonged ventrally and dorsally into two cæcal appendages resembling the single appendage of the pig’s stomach. (Columbia University Museum, No. 1806.)
5. Variations in the Form of the Stomach Depending upon the Assumption by the Stomach of Special Functions, which are Usually Relegated to other Organs.—These functions are the following:
| Fig. 80.—Macacus nemestrinus, pig-tail macaque monkey; cheek-pouches. (From a fresh dissection.) |
| Fig. 81.—Stomach of Cricetus vulgaris, hamster. (Nuhn.) |
(a) Storage of food in special receptacles or compartments for subsequent use.
(b) Mastication of the food is in some animals accomplished only partly or not at all in the mouth, and is then performed in the stomach. A portion of the stomach is thus converted into an apparatus for mastication.
(c) The provisions for these two accessory functions may be combined in the same stomach.
(a) Many of the higher vertebrates possess in connection with the alimentary tract additional reservoirs for the storage of food until used. Such reservoirs are found in mammals and birds connected with the oral cavity, as cheek-pouches, or with the œsophagus, such as the crop of the birds ([Fig. 88]). Fig. 80 shows the development of the cheek-pouches in one of the primates, Macacus nemestrinus.
In many mammals reservoirs of similar import are added directly to the stomach and form an integral part of the organ. Examples are furnished by the compound stomachs of many rodents, ruminants, cetaceans and herbivorous edentates. The peculiar appearance of these stomachs is explained if the additional reservoirs are in imagination removed and the digestive stomach proper restored so to speak to the type-form. The proximal or cardiac portion of the stomach in many rodents is devoid of gastric glands and must be interpreted as a storage chamber for food (Fig. 81). The same significance attaches to the corresponding portion of the manatee’s stomach ([Fig. 68]).
Similar contrivances are found in the ruminant stomach. The first and second divisions (rumen and reticulum) are nothing but sac-like gastric reservoirs or pouches, in which the food is collected, to be subsequently returned to the mouth for mastication. When swallowed for the second time the bolus is carried, by the closure of the so-called œsophageal gutter, past the first and second stomach into the digestive apparatus proper (the abomasum) (Figs. 82 and 83). Many ruminants (e. g., Moschus) only have these three compartments. Most, however, have four, the leaf stomach or psalterium being intercalated between the retinaculum and the abomasum. The psalterium contains no digestive glands. It may possibly serve for the absorption of the liquid portions of the foods.
The rumen or first stomach of the camels and llamas is provided with so-called “water-cells,” for the storage of water. These cells are diverticula lined by a continuation of the gastric mucous membrane. The entrance into these compartments can be closed by a sphincter muscle after they are filled with water (Fig. 84).
Fig. 86.—Stomach of Urinator imber, red-throated loon. (Columbia University Museum, No. 1808.)
Fig. 87.—Scheme of stomach of granivorous bird. (Nuhn.)
The three stomachs of the cetaceans are similar to those of the ruminants (Fig. 85). The first is a crop-like reservoir for the reception of the food when swallowed. The mucous membrane is entirely devoid of digestive glands. In the dolphins the mucous membrane is provided with a hard horny covering, which serves to break up the food mechanically by trituration. The second stomach and the gut-like pyloric prolongation constituting the third stomach contain gastric glands and are hence digestive in function.
(b) Stomach forms, in which a portion of the organ is converted into an apparatus for mastication, are seen especially in birds, in which animals, on account of the absence of teeth, mastication cannot be performed in the mouth.
The stomach of the bird is usually composed of two segments, one placed vertically above the other.
The first appears like an elongated dilatation of the œsophagus, forming the Proventriculus or glandular stomach.
The second is larger, round in shape, with very strong and thick muscular walls (Figs. 86 and 87).
The proventriculus furnishes the gastric juice exclusively.
The second or muscular stomach, devoid of gastric glands, functions merely as a masticating apparatus for the mechanical division of the food. The thick muscular walls of this compartment may measure several inches in diameter and carry on the opposed mucous surfaces lining the cavity a hard horny plate with corrugated and roughened surface (Fig. 88). These hard plates are designed to crush the food between them, as between two mill stones. The muscle stomach is best developed in herbivorous birds, while both the muscular wall and the horny plate are much weaker and thinner in carnivore wading and swimming birds (Fig. 89).
| Fig. 88.—Œsophagus and stomach of Gallus bankiva, hen. (Columbia University Museum, No. 1809.) | Fig. 89.—Stomach of Botaurus lentiginosus, bittern. (Columbia University Museum, No. 23/1810.) |
| Fig. 90.—Stomach of owl sp. (Nuhn.) | Fig. 91.—Stomach of Ardea cinerea, heron. (Nuhn.) |
Fig. 92.—Stomach of crocodile. (Nuhn.)
In birds of prey, especially in the owls, the stomach walls are scarcely more massive than in other animals, and the mucous membrane is soft and devoid of a horny covering. The glandular and masticatory stomachs are less sharply divided from each other in these forms, and the entire organ conforms more to the general vertebrate type (Fig. 90).
In some birds (herons, storks, etc.) a small rounded third stomach, the so-called pyloric stomach, is placed between the muscle stomach and the pylorus (Fig. 91). It contains no gastric glands, and possibly may function as an additional absorbing chamber.
Among reptiles the stomach of the crocodile resembles the organ in birds (Fig. 92). It is flat and rounded in shape, the muscle wall carries a tendinous plate, and there is a pyloric stomach. There is, however, no glandular stomach or proventriculus, as in birds, and the mucous membrane is not covered by a horny plate, but is soft and contains the peptic glands. Figs. 93 and 94 show the stomach of Alligator mississippiensis, in the ventral view and in section.
| Fig. 93.—Stomach of Alligator mississippiensis. (Columbia University Museum, No. 1811.) | Fig. 94.—Same in section. Thin-walled cardiac segment continues into cavity of pyloric ventriculus. |
Fig. 95.—Stomach of Bradypustridactylus, three-toed sloth. I. First stomach, devoid of gastric glands, corresponding to rumen of ruminants.
II. Second stomach, the homologue of the ruminant reticulum.
III. Digestive stomach proper, provided with gastric glands connected by a gutter with the œsophagus.
IV. Muscular stomach, the walls formed by a thick muscular plate and provided on the mucous surface with a dense corneous covering for purposes of trituration.
(c) The combination of the two accessory functions just described in the same stomach is found in the three-toed sloth (Fig. 95).
There are here two large reservoirs, which correspond to the rumen and retinaculum of the ruminants, and a digestive compartment containing gastric glands, which corresponds to the ruminant abomasum, and is connected by an œsophageal gutter directly with the œsophagus. At the pyloric extremity the muscle wall is greatly increased and the mucous membrane of this portion carries a thick horny covering, forming a masticatory stomach greatly resembling the corresponding structure in the bird. Its function is evidently to complete the mechanical division of the food which has only been partly masticated in the mouth.
The same significance is probably to be attached to the thickened muscular walls which the pyloric segment of the stomach in Tamandua bivittata, another edentate, presents (Fig. 96), in strong contrast with the thinner walled cardiac segment and fundus.
Fig. 96.—Stomach of Tamandua bivittata, collared ant-eater, (Columbia University Museum, No. 68/1485.)