The pharynx includes the stretch of digestive highway from the posterior part of the mouth cavity to the beginning of the esophagus. While its actual extent is relatively small, its diversity of function, and consequently the degree to which it is modified in various vertebrates, is very great.
The pharynx serves as a point of departure for describing the respiratory system, since among fishes and amphibians it is the region of the gills, as well as the point of origin for the swim bladder of fishes and the lungs of land animals. This aspect of the pharynx will be taken up in the chapter on Respiration. The description of the numerous pharvngeal glands, thyroid, parathyroids, thymus, epithelial corpuscles, and ultimobranchials, will also be deferred until the chapter on endocrinal glands.
Traveling through the gateway of the pharynx are two quite different streams of material, namely food and oxygen. In fishes they enter the mouth together (Fig. 251a), and proceed in parallel course without mutual interference, food passing straight to the esophagus where it continues on its way, while oxygen, dissolved in water, passes out over the gills hanging in the lateral gill slits, which like portholes pierce the sides of the pharynx. The paired nasal pits on the snout of a fish do not open into the mouth cavity, and have nothing to do with the pharynx or with breathing.
In amphibians, the first land forms that possess lungs and breathe free air, the nasal pits deepen until they break through into the mouth cavity, thereby forming a pair of respiratory passage-ways. The openings into the mouth cavity are the internal nares, or choanae (Fig. 251b). These allow air to pass to the lungs without the necessity of opening the mouth, thereby exposing its mucous lining to disastrous drying up. Free air is taken into the mouth cavity through the choanae with the mouth closed. After valves in the external nares have closed, the floor of the mouth is raised. Thus the air is forced back into the lungs, a process which would be quite impossible with the mouth open, as the air could then escape in the wrong direction.
Embryonically the lungs are ventral outgrowths from the floor of the pharynx, and thus, while the food takes a straight course from mouth to esophagus as in fishes, air, entering the nostrils dorsally, crosses the path of the food and is forced ventrally into the lungs.
Vertebrates above the amphibians, that is, reptiles, birds, and mammals, have developed a hard palate, or secondary roof of the mouth, which forces the choanae backward so that the crossing of the ways is transferred from the oral cavity to the pharynx. This crossing is known as the pharyngeal chiasma.
In mammals this arrangement has been accompanied by the establishment of various anatomical modifications (Fig. 252) that, like traffic officers at a busy street crossing, regulate the traffic and prevent confusion, although adding materially to the expense of maintenance. The epiglottis, for example, is introduced as a trapdoor device to guard the entrance into the trachea so that food passing by shall not go the wrong way.
It may be pointed out that if in man air had been made to pass through the chin instead of the nose, thus avoiding the pharyngeal chiasma entirely, at least some present troubles would have been eliminated, although with such a drastic change other difficulties might have been introduced. In any case the pharyngeal chiasma clearly stresses the fact that our anatomical machinery is the inevitable result of many successive modifications rather than being formed de novo from blueprint specifications. A moral to be gained from comparative anatomy, as well as from other aspects of life, is that one should seek to make the best of his inheritance, whatever it is, rather than vainly to regret not having been endowed with perfection in the beginning. The acquisition of adaptations by successive stages, each of which is always dependent upon what has gone before, is not only the method invariably followed in the phylogenetic establishment of species, but it is also the only way that any anatomical structure in an individual comes into being embryonically.
In man the pharynx is shaped somewhat like a funnel about five inches deep. It extends from the base of the skull to the level of the sixth cervical vertebra, where it narrows into the esophagus. Its three merging irregular cavities, one below the other, may, for purposes of description, be designated as the nasopharynx, oropharynx, and laryngopharynx.
The upper nasopharynx (Fig. 252), which is not concerned with alimentary traffic but is entirely respiratory in function, lies dorsal to the soft palate. In general it retains a definite contour, since its bony walls are practically inflexible. On either side of the nasopharynx opens an Eustachian tube from the air chamber of the middle ear. These tubes are the morphological successors of the second pair of gill slits, or the spiracles, between the mandibular and hyoid arches of ancestral aquatic forms.
The oropharynx, posterior to the nasopharynx, communicates through the isthmus of the fauces, or the posterior exit of the oral cavity, directly with the cavity itself. Forming the ventral wall of the oropharynx is the base of the tongue that here assumes a vertical position, practically reducing the oropharynx to a transverse slit when the mouth is closed.
On the sides of the oropharynx are two masses of glandular or lymphoid tissue, the palatine tonsils, while upon the posterior wall of the nasopharynx is still more of this peculiar tissue, the pharyngeal tonsils, commonly known when enlarged as adenoids. On the vertical face of the base of the tongue, behind the sulcus terminalis, are the lingual tonsils. An incomplete ring of tonsillar tissue, therefore, surrounds the pharyngeal passageway, of which the part made up of palatine tonsils is the most prominent.
Along with the tonsils there is developed throughout the entire pharyngeal region a variety of adaptive glandular and lymphoid structures having a wide range of functions, and forming the seat of so many complications and troubles, both structural and physiological, that physicians and surgeons specializing in this field alone, have their busy hands full.
The laryngopharynx, continuous with the oropharynx, is the indefinite lower part of the pharynx between the soft palate and the esophagus. It includes the critical region of the pharyngeal chiasma, and surrounds the larynx, or voice box. Except during the passage of food, which slips past down either side of the closed glottis, the laryngopharynx is collapsed into a narrow slit.
Thus it will be seen that the pharynx as a whole, like a colonial kitchen, opens into several adjoining spaces. It has communication in fact through seven different openings, namely, two choanae, and two Eustachian tubes in the nasopharynx; the isthmus of the fauces in the oropharynx; and in the laryngopharynx, the glottis and the esophageal opening.
Over the entrance of the esophagus might well be written Dante’s immortal line: “All hope abandon, ye who enter here,” for in its course the muscles of its walls pass from voluntary to involuntary nerve control. Of all the vertebrates, some birds and the ruminant mammals are exceptional in that they have voluntary muscle fibers extending the whole length of the esophagus and are, therefore, able to regurgitate the food at will. The so-called “milk” which some birds regurgitate is used to feed their young. Ruminants, on the other hand, regurgitate their hastily swallowed food for more prolonged chewing.
The esophagus, a short comparatively unmodified part of the digestive tube between the pharynx and the stomach (Fig. 253), is primarily a sphincter, the office of which is to forward food by peristalsis along its course to a point beyond normal control. The peristaltic action of the walls of the esophagus is well shown by a horse drinking at a brook, for the gulps of water taken in have to travel up hill along the neck and their passage is externally visible. In the case of a snake the violent peristalsis necessary in swallowing a comparatively large morsel of food, such as a frog, is supplemented by the muscles of the body wall.
When not in use the esophagus collapses to modest dimensions, but upon occasion it is capable of great temporary distension. There are certain fishes that can even swallow another fish larger than themselves (Fig. 254). In many vertebrates the inner lining of the esophagus is characterized by expansive longitudinal folds that allow for a sudden increase in diameter during the act of swallowing, but at other times contract so that the tube may occupy a minimum of space.
The inner lining of the esophagus of marine turtles is beset with backward-projecting horny papillae, which enable them more easily to swallow the slippery seaweeds upon which they habitually feed.
The length of the esophagus is dependent largely upon the presence or absence of a neck. In frogs and toads the neck is reduced to a minimum so that a fly entering the mouth of one of these animals finds itself almost immediately landed in the stomach, whereas in long-necked animals, such as the giraffe for example, the esophageal adventures of food are much more extended. In adult man the length of the esophagus is approximately fourteen inches, the lower end piercing the diaphragm to enter the body cavity before joining the stomach. It is only this short portion within the body cavity that is provided with a serosa layer of tissue outside the muscular and mucosa layers.
A noteworthy differentiation of the esophagus in birds is a lateral enlargement known, as the crop (Fig. 255), which may serve simply as a convenience for the temporary storage of food hastily secuied in the presence of enemies or competitors, as in the case of seed-eaters generally, or may be supplied with glands which act chemically upon the food eaten. Pigeons produce a cheesy nutritious substance from the lining of the crop, called “pigeon’s milk,” that is fed to nestlings by regurgitation. The tropical hoactzin, Opisthocomus, has a muscular crop which works mechanically upon the food that finds lodgment in it. A chicken going to roost with its crop filled with corn, falls asleep unhampered by the continuous effort of picking up food and “feeds” all night long while resting, as the crop, like the hopper of a gristmill, releases its contents automatically and periodically to the glandular stomach and grinding gizzard as needed.
Among the lower vertebrates any external line of demarcation between the esophagus and stomach is either absent or vague, but in birds and mammals there is usually a definite point of transition. In many cases it is easier to gain entrance to the stomach from the esophagus than to escape from the stomach into the intestine.
The stomach is a conspicuous enlargement of the digestive tract lying between the esophagus and the intestine (Fig. 256). Originally, as in some fishes and salamanders, it is spindle-shaped and arranged to conform with the general contour of an elongated body, but in higher vertebrates it becomes saclike in shape, assuming a somewhat transverse position in the body cavity. Between these extremes may be found many gradations of form and position.
The stomach of the dogfish, for example, instead of being a primitive straight spindle-shaped enlargement with the entrance and exit at opposite ends, is doubled back in the form of a J-shaped tube. Stomachs of similar shape occur also in the flounder, haddock, salmon, carp, sturgeon, sole, and many other fishes. In fishes such as the perch, smelt, herring, bullhead, and whiting, the loop becomes fused along its inner bend in such a way that a bag-shaped pouch, or fundus, is formed with the entrance and exit brought near together at one side. This type of a stomach, when shifted into a transverse position, is much like that of man, with a lesser curvature on the upper side between the entrance and the exit, and a greater curvature forming the larger contour around the outer margin or elbow of the stomach.
The entrance to the human stomach is somewhat larger than the exit and is less distinctly marked off, although the lining of the digestive tract itself in the region of the esophagus is easily distinguished from that of the stomach, even when the external transition from one part to the other is extremely vague and indefinite.
The exit from the stomach is closed by the pylorus, or pyloric valve, a fold of mucous membrane reinforced by a sphincter muscle, which relaxes temporarily for the release of food into the intestine only when the stimulative password of proper acidulation is given.
The walls of the stomach are muscular enough to insure the active movement of the food-mass around and around by peristalsis until it has been reduced, through mixture with glandular secretions, to a suitable consistency and degree of acidity. In other words, the food is kneaded and mixed under muscular pressure. Then, as it is presented at the closed pylorus, the sphincter muscle relaxes, allowing small successive amounts of the mixture, called chyme, to slip through into the intestine.
Amphioxus and larval cyclostomes, which have not gone far enough in evolution to develop peristaltic muscles, have, according to Schimkewitsch, the entire digestive tube lined with cilia, the mission of which is to keep the food moving along. In the human fetus also, as a possible reminder of whence man came, the posterior part of the stomach lining is clothed with cilia.
There is a tendency for the stomach to become differentiated into two or more regions, distinguished from each other by location and function. Thus, in the J-shaped stomach of the dogfish which involves half the entire length of the digestive tract one speaks of a cardiac limb and a pyloric limb, while in certain mammals, the mouse for example, a constriction in the middle part of the stomach marks off a cardiac chamber from a pyloric chamber.
Medical literature contains references to the occasional occurrence of so-called “hour-glass stomachs” in man (Fig. 257), which bear a strong resemblance to the two-chambered stomachs of mice. Certain monkeys (Hylobates and Semnopithecus) show the same feature. Whether such unusual structures in m?n are pathological or ancestral is uncertain.
An extreme subdivision of the stomach is reached by the ruminants, which have four “stomachs” (Fig, 258). The first in order is the “paunch,” or rumen, which is a spacious storage bag for the temporary reception and fermentation of grass or herbage upon which ruminants feed. Microorganisms present in the rumen of domestic cattle, and possibly other ruminants, act upon simple nitrogenous compounds to synthesize proteins as well as sufficient quantities of the B-complex vitamins to supply the dietary needs of these vertebrates. From the rumen the food is passed over into the “honeycomb stomach,” or reticulum, that, as its name indicates, is lined with many shallow pits. When leisure from prehensile feeding comes, food which was hurriedly swallowed with little mastication is regurgitated into the mouth for rechewing. This material, coming in part from the rumen and in part from the reticulum, is known as the “cud.” It includes roughage, such grains as happen to be trapped in the roughage, and a considerable quantity of water which facilitates the passage of the cud up the esophagus. During the first few chewing movements the animal swallows most of the liquid brought up. After remastication and a thorough mixing with saliva, the food is again swallowed and passes once more into the rumen. Then another cud is regurgitated, thus beginning a new cycle of rumination. Most of the food that has been thoroughly chewed and mixed with liquid soon passes into the reticulum and then shortly into the omasum, or “manyplies stomach.” This third chamber is lined with numerous folds and communicates directly with the true “glandular stomach,” or abomasum, where the food is mixed with gastric juices and chemically modified before being forwarded into the intestine. In “water cells” in the walls of the rumen and reticulum, camels are able to store reserve water which enables these desert animals to endure prolonged periods of dryness.
The vampire bat, Desmodus, exhibits a peculiar adaptation with reference to its blood-sucking habits, the fundus of the stomach being drawn down into a deep elastic pouch (Fig. 256). When a vampire fastens on to a warm-blooded victim it can fill this spacious reservoir with blood until the entire body is swollen in consequence.
The cardiac and pyloric regions of the stomach in birds have become separated into chambers very unlike each other in character (Fig. 255). The cardiac chamber, or proventriculus, which opens into the gizzard, becomes a glandular stomach, where the food undergoes some preliminary maceration and chemical modification before reaching the gizzard through which it passes to the intestine. The gizzard has a thick muscular wall and is lined with a hard secreted layer. In this muscular mill food is ground up, instead of by means of teeth as in mammals. Gravel, or “gizzard stones,” retained temporarily within the gizzard cavity, aids in the process of food attrition. The whole device is a part of the general program of centralization of organs which the birds, as adapted flying machines, have evolved. The highest differentiation of the gizzard is reached in seed-eating birds, and the least in birds of prey.
Among reptiles the crocodilian stomach approaches that of birds in its differentiation, since a gizzard-like pyloric chamber receives the food after it passes over from the glandular cardiac sac, which corresponds to the chemically functioning stomach of other animals.
There are at least three general functions performed by the vertebrate stomach, namely, storage, mechanical manipulation, and chemical modification.
The advantages of food storage are obvious. Among lower sedentary creatures, like sponges and clams for example, there is no provision for food storage, in consequence of which feeding is practically a continuous process. With the necessity of hunting for food, rivalry for daily bread and adventurous escape from devouring enemies become more and more the constant daily program of animal existence. The necessity for seizing a sufficient supply of food, when it is available, in a minimum of time and then retiring to safety or engaging in other activities, is apparent. By periodic voluntary filling of a storage chamber like the stomach with food, opportunity is left for other activities at the same time that the involuntary machinery of the body is faithfully attending to the digestive processes with meticulous deliberation and care.
The function of mechanical manipulation, or peristalsis, has already been mentioned. By this means the muscular walls of the stomach knead the food-mass around, mixing it with digestive secretions. The movement may actually be seen upon a fluorescent screen when an animal like a cat, whose food has been mixed with barium sulphate or some other substance that is opaque to X-rays, is exposed to their action (Fig. 259).
The function of chemical modification is dependent upon the presence of glands in the lining of the stomach, which produce secretions of various kinds. In the region of the fundus, gastric glands are most numerous. These produce hydrochloric acid and three kinds of enzymes, namely, pepsin, rennin, and gastric lipase, which do preliminary service in the chemical reduction of proteins and fats. Pepsin, acting only in an acidulated medium, breaks down protein foods to simpler compounds; rennin coagulates the protein casein, a constituent of cheese, out of milk, rendering it capable of being changed by pepsin into simpler substances that are prepared to undergo other necessary changes farther along in the digestive tract; finally, gastric lipase begins the work of splitting up fats into soluble fatty acids and glycerine.
It is the hydrochloric acid generously produced by the gastric glands of dogs that enables them to dissolve bones which they crunch and swallow.
The protein lining of the stomach itself is not digested by its own secretions because its component cells are living and thus resistant to enzymatic action. It is only when a cell is dead that it yields to destruction by gastric juices. This explains how a tapeworm can live and prosper while bathed in the digestive gastric secretions of its host.
Man is the only animal that hastens the reduction of food to soluble form by cooking it.