When we try to construct an animal frame of the simplest conceivable type, that has some such primitive alimentary canal and the two primary layers constituting its wall, we inevitably come to the very remarkable embryonic form of the gastrula, which we have found with extraordinary persistence throughout the whole range of animals, with the exception of the unicellulars—in the Sponges, Cnidaria, Platodes, Vermalia, Molluscs, Articulates, Echinoderms, Tunicates, and Vertebrates. In all these stems the gastrula recurs in the same very simple form. It is certainly a remarkable fact that the gastrula is found in various animals as a larva-stage in their individual development, and that this gastrula, though much disguised by cenogenetic modifications, has everywhere essentially the same palingenetic structure (Figures 1.30 to 1.35). The elaborate alimentary canal of the higher animals develops ontogenetically from the same simple primitive gut of the gastrula.
This gastraea theory is now accepted by nearly all zoologists. It was first supported and partly modified by Professor Ray-Lankester; he proposed three years afterwards (in his essay on the development of the Molluscs, 1875) to give the name of archenteron to the primitive gut and blastoporus to the primitive mouth.
Before we follow the development of the human alimentary canal in detail, it is necessary to say a word about the general features of its composition in the fully-developed man. The mature alimentary canal in man is constructed in all its main features like that of all the higher mammals, and particularly resembles that of the Catarrhines, the narrow-nosed apes of the Old World. The entrance into it, the mouth, is armed with thirty-two teeth, fixed in rows in the upper and lower jaws. As we have seen, our dentition is exactly the same as that of the Catarrhines, and differs from that of all other animals (Chapter 2.23). Above the mouth-cavity is the double nasal cavity; they are separated by the palate-wall. But we saw that this separation is not there from the first, and that originally there is a common mouth-nasal cavity in the embryo; and this is only divided afterwards by the hard palate into two—the nasal cavity above and that of the mouth below (Figure 2.311).
At the back the cavity of the mouth is half closed by the vertical curtain that we call the soft palate, in the middle of which is the uvula. A glance into a mirror with the mouth wide open will show its shape. The uvula is interesting because, besides man, it is only found in the ape. At each side of the soft palate are the tonsils. Through the curved opening that we find underneath the soft palate we penetrate into the gullet or pharynx behind the mouth-cavity. Into this opens on either side a narrow canal (the Eustachian tube), through which there is direct communication with the tympanic cavity of the ear (Figure 2.320 e). The pharynx is continued in a long, narrow tube, the oesophagus (sr). By this the food passes into the stomach when masticated and swallowed. Into the gullet also opens, right above, the trachea (lr), that leads to the lungs. The entrance to it is covered by the epiglottis, over which the food slides. The cartilaginous epiglottis is found only in the mammals, and has developed from the fourth branchial arch of the fishes and amphibia. The lungs are found, in man and all the mammals, to the right and left in the pectoral cavity, with the heart between them. At the upper end of the trachea there is, under the epiglottis, a specially differentiated part, strengthened by a cartilaginous skeleton, the larynx. This important organ of human speech also develops from a part of the alimentary canal. In front of the larynx is the thyroid gland, which sometimes enlarges and forms goitre.
The oesophagus descends into the pectoral cavity along the vertebral column, behind the lungs and the heart, pierces the diaphragm, and enters the visceral cavity. The diaphragm is a membrano-muscular partition that completely separates the thoracic from the abdominal cavity in all the mammals (and these alone). This separation is not found in the beginning; there is at first a common breast-belly cavity, the coeloma or pleuro-peritoneal cavity. The diaphragm is formed later on as a muscular horizontal partition between the thoracic and abdominal cavities. It then completely separates the two cavities, and is only pierced by several organs that pass from the one to the other. One of the chief of these organs is the oesophagus. After this has passed through the diaphragm, it expands into the gastric sac in which digestion chiefly takes place. The stomach of the adult man (Figure 2.349) is a long, somewhat oblique sac, expanding on the left into a blind sac, the fundus of the stomach (b apostrophe), but narrowing on the right, and passing at the pylorus (e) into the small intestine. At this point there is a valve, the pyloric valve (d), between the two sections of the canal; it opens only when the pulpy food passes from the stomach into the intestine. In man and the higher Vertebrates the stomach itself is the chief organ of digestion, and is especially occupied with the solution of the food; this is not the case in many of the lower Vertebrates, which have no stomach, and discharge its function by a part of the gut farther on. The muscular wall of the stomach is comparatively thick; it has externally strong muscles that accomplish the digestive movements, and internally a large quantity of small glands, the peptic glands, which secrete the gastric juice.
(FIGURE 2.349. Human stomach and duodenum, longitudinal section. a cardiac (end of oesophagus), b fundus (blind sac of the left side), c pylorus-fold, d pylorus-valves, e pylorus-cavity, fgh duodenum, i entrance of the gall-duct and the pancreatic duct. (From Meyer.)
FIGURE 2.350. Median section of the head of a hare-embryo, one-fourth of an inch in length. (From Mihalcovics.) The deep mouth-cleft (hp) is separated by the membrane of the throat (rh) from the blind cavity of the head-gut (kd). hz heart, ch chorda, hp the point at which the hypophysis develops from the mouth-cleft, vh ventricle of the cerebrum, v3, third ventricle (intermediate brain), v4 fourth ventricle (hind brain), ck spinal canal.)
Next to the stomach comes the longest section of the alimentary canal, the middle gut or small intestine. Its chief function is to absorb the peptonised fluid mass of food, or the chyle, and it is subdivided into several sections, of which the first (next to the stomach) is called the duodenum (Figure 2.349 fgh). It is a short, horseshoe-shaped loop of the gut. The largest glands of the alimentary canal open into it—the liver, the chief digestive gland, that secretes the gall, and the pancreas, which secretes the pancreatic juice. The two glands pour their secretions, the bile and pancreatic juice, close together into the duodenum (i). The opening of the gall-duct is of particular phylogenetic importance, as it is the same in all the Vertebrates, and indicates the principal point of the hepatic or trunk-gut (Gegenbaur). The liver, phylogenetically older than the stomach, is a large gland, rich in blood, in the adult man, immediately under the diaphragm on the left side, and separated by it from the lungs. The pancreas lies a little further back and more to the left. The remaining part of the small intestine is so long that it has to coil itself in many folds in order to find room in the narrow space of the abdominal cavity. It is divided into the jejunum above and the ileum below. In the last section of it is the part of the small intestine at which in the embryo the yelk-sac opens into the gut. This long and thin intestine then passes into the large intestine, from which it is cut off by a special valve. Immediately behind this "Bauhin-valve" the first part of the large intestine forms a wide, pouch-like structure, the caecum. The atrophied end of the caecum is the famous rudimentary organ, the vermiform appendix. The large intestine (colon) consists of three parts—an ascending part on the right, a transverse middle part, and a descending part on the left. The latter finally passes through an S-shaped bend into the last section of the alimentary canal, the rectum, which opens behind by the anus. Both the large and small intestines are equipped with numbers of small glands, which secrete mucous and other fluids.
For the greater part of its length the alimentary canal is attached to the inner dorsal surface of the abdominal cavity, or to the lower surface of the vertebral column. The fixing is accomplished by means of the thin membranous plate that we call the mesentery.
Although the fully-formed alimentary canal is thus a very elaborate organ, and although in detail it has a quantity of complex structural features into which we cannot enter here, nevertheless the whole complicated structure has been historically evolved from the very simple form of the primitive gut that we find in our gastraead-ancestors, and that every gastrula brings before us to-day. We have already pointed out (Chapter 1.9) how the epigastrula of the mammals (Figure 1.67) can be reduced to the original type of the bell-gastrula, which is now preserved by the amphioxus alone (Figure 1.35). Like the latter, the human gastrula and that of all other mammals must be regarded as the ontogenetic reproduction of the phylogenetic form that we call the Gastraea, in which the whole body is nothing but a double-walled gastric sac.