CHAPTER VI
DIGESTION
The Canal.—The prospect presented by a widely open mouth is too familiar to need description, but a few details may be pointed out. The teeth are, or should be, thirty-two in number. Starting from the middle line of either jaw, the two first are incisors, with chisel-shaped cutting edges. If they meet, as they ought to do, their edges are ground flat. The third tooth is the canine, with a more or less pyramidal crown. Then two premolars, or “milk-molars,” as they are often termed, because they are the only grinding teeth of the first dentition. Twenty is the full complement of teeth in a child. Lastly, three strong grinders—the molar teeth. The third molar, or wisdom-tooth, is evidently disappearing in the human race. In civilized people, whose brains are large and jaws small, it does not appear until about the twentieth year. Sometimes it tries to squeeze through the gum of a jaw not large enough to carry it, and causes trouble by becoming “impacted” beneath the ascending ramus. Not infrequently it fails to appear. It may be truly said that the increasing wisdom of the human race is responsible for the postponement of its development, although this is hardly the circumstance to which it owes its name. A fold of mucous membrane—the frenulum linguæ—connects the under side of the tongue with the floor of the mouth. On either side of this may be seen the opening of a duct common to the submaxillary and sublingual salivary glands. The opening of the duct of the parotid gland is not so easy to find. It pierces the mucous membrane of the cheek opposite to the base of the second molar tooth of the upper jaw. The parotid gland lies just below the ear, behind the jaw. The saliva which it secretes is a watery fluid containing little beside salt and a weak ferment. It serves to moisten the food as it is being crushed by the molar teeth. The submaxillary and sublingual secretions contain, in addition to the ferment, ptyalin, mucus which the tongue mixes with the masticated food as it forms it into a bolus suitable for swallowing. The dorsal surface of the tongue is covered by papillæ, which rasp the food against the palate. Of these the greater number are pointed, or filiform. The remainder are flat-topped, or fungiform. The two varieties may be distinguished with a lens, especially on the sides of the tongue. Usually the fungiform papillæ are the redder. In fever, when the tongue is densely furred, they stand out as bright red spots. The back of the tongue is crossed by a V-shaped row of papillæ of larger size, each surrounded by a slight fossa and a vallum, and hence termed “circumvallate.” Very minute organs of sense—taste-bulbs—stud the mucous membrane which lines the fosse.
The hard palate ends in a muscular curtain—the soft palate—the central portion of which—the uvula—depends lower than the rest. On either side the soft palate splits into two folds; the anterior, continued to the side of the tongue; the posterior, to the pharynx. These folds, since they bound the gateway into the pharynx, which is known as the “fauces,” are termed the “pillars of the fauces.” The tonsil lies between the anterior and posterior pillars of the fauces, but does not appear as a prominence unless inflamed or enlarged.
The pharynx hangs as a bag from the base of the skull. It, like all the rest of the alimentary tract, is lined by mucous membrane. “Mucous membrane” is not a happy term. It does not denote that the epithelium secretes mucus. It may or may not possess this property. Nor does it imply that it has a different origin from the skin—that it arises from hypoblast, the inner layer of the rudiment from which the embryo grows. The term is applied to all internal, and therefore moist, surfaces, whether they arise from hypoblast, as in the case of the lining of the greater part of the alimentary tract, or whether they are involutions of epiblast as in the case of the mouth and also of the extreme lower end of the alimentary tract. Almost the whole of the alimentary canal is, in the first instance, a tubular cavity in the interior of the embryo, lined by hypoblast. This cavity communicates with the yolk-sac, but has no openings on the exterior until it joins up with two epiblastic pits—one the stomodæum, or mouth-cavity, at the anterior end; and the other the proctodæum, at the posterior end of the body. The distinction between the middle closed portion of the alimentary canal and its two secondary openings suggests morphological speculations, into which we have not space to enter, as to the ancestry of the vertebrates. The majority of anatomists believe that the primitive canal is represented in the middle portion, and that, in prevertebrate animals, it opened to the exterior in a different way. The pharynx is 4½ inches long. It is enclosed by three thin muscles, which overlap from below upwards—the constrictors of the pharynx. The anterior attachment of the superior constrictor is to the jaw; of the middle constrictor to the hyoid bone; of the inferior constrictor to the thyroid cartilage. Above the soft palate the nasal chambers communicate with the pharynx by the posterior nares. Below the hyoid bone, which is easily felt in the neck as a bony arch just above the thyroid cartilage (Adam’s apple), the windpipe, or trachea, joins the pharynx by a single pear-shaped orifice—the rima glottidis. When we consider the mechanism of swallowing, we shall study the arrangements which prevent food, passed through the fauces, from entering either the nasal chambers above or the windpipe below and in front. At the level of the lower border of the thyroid cartilage the pharynx becomes the relatively narrow œsophagus. This tube, which lies behind the trachea, and slightly to its left side, passes with a straight course to the abdomen. It traverses the chest, lying behind the heart, pierces the diaphragm, and just beneath it joins the stomach. Its length is about 9 inches. The stomach is a sickle-shaped bag. It has two apertures—the cardiac orifice, or junction with the œsophagus; and the pyloric orifice, or junction with the small intestine. It is so folded on itself that these two apertures are not more than 4 inches apart. Its outline may be drawn on the body-wall with a piece of charcoal from a point an inch below and an inch to the left side of the lower end of the breast-bone, the position of the cardiac orifice, to a point about 4 inches below the end of the breast-bone, and an inch or two to the right side of the mid-line of the body, the position of the pyloric orifice, with a slight curvature to represent the upper border; to represent the lower border the same two points are joined by a bold curve, bulging upwards to the nipple, outwards to the side of the body, and downwards some distance on the abdomen ([cf. Fig. 2]).
Fig. 6.
The stomach has been cut across a short distance from the pyloric valve, and removed, to show the viscera which lie behind it. The descending aorta and the vena cava rest upon the vertebral column. They are crossed by the pancreas and the transverse portion of the duodenum. The head of the pancreas is enclosed by the curvatures of the duodenum. The ducts of the liver and pancreas are seen entering the descending duodenum side by side.
Such an outline represents the form and position of the stomach when distended; but it is to be understood that its dimensions depend upon the amount of its contents. It is capable of holding about 7 pints. The junction of œsophagus and stomach is closed by a muscular ring, or sphincter muscle—the cardiac sphincter; the junction of stomach and intestine is guarded by a much stronger pyloric sphincter. The average diameter of the small intestine is about 1½ inches. It is wide enough, therefore, to admit two fingers. The length of the tube is about 22 feet. Its first part is termed the “duodenum,” because its length equals the breadth of twelve fingers—i.e., about 9 inches. The remainder is divided arbitrarily into jejunum and ileum. The duodenum makes three sharp curves. First it inclines upwards and to the right, then vertically downwards, then horizontally to the left, and finally forwards. The ducts of the liver and pancreas open by a common orifice into the descending portion. Its horizontal portion is bound firmly to the vertebral column. After this the whole of the small intestine is supported by the mesentery, a double fold of peritoneum which allows it to hang freely in the abdominal cavity. The mesentery is attached to the back of the body-wall. Commencing on the left side of the second lumbar vertebra, its line of attachment inclines obliquely downwards and to the right, across the vertebral column, for about 6 inches. Measured from its attached edge to the edge which bears the intestine, it has a width of about 8 inches. Its free border has, as already said, a length of 22 feet. Its measurements being as just stated, it is clear that it must be folded backwards and forwards upon itself, like a goffered frill. In the right groin the small intestine joins the large intestine, or colon. It does not, as might have been expected, simply dilate into the large intestine, but enters it on its mesial side, its orifice being guarded by the ileo-colic valve. In other words, the large intestine projects downwards beyond this orifice, as the cæcum coli. In many animals the cæcum is of great length and capacity. In the human embryo it begins to assume a similar form; but a very small portion only (the so-called “cæcum” of human anatomy) dilates to the calibre of the colon. The real cæcum retains throughout life its embryonic calibre. It has a length of about 3½ inches, and a diameter of not more than ¼ inch. This is the “vermiform appendix,” of ill fame, which must be looked upon as one of Nature’s misfits. Its great liability to become inflamed is commonly explained as due to the tendency of such articles of food as pips, the fibre of ginger, flakes from the inside of enamelled saucepans, etc., to become lodged in its cavity. But whether this explanation be correct or no—and there are reasons for thinking it somewhat fanciful—it is much to be wished that the process of evolution would hasten the disappearance of this functionless vestige of a cæcum. As there is no tendency towards the inheritance of characters due to mutilation, and since the surgeon’s knife now prevents this death-trap from claiming its toll of possible parents, we must look upon the rudimentary cæcum, with its liability to inflammation, as a permanent burden on the human race. In justice to the appendix, however, it must be pointed out that it has acquired its criminal reputation during the past twenty years. The frequency of appendicitis has increased so enormously during this period that it ought to be possible to correlate its prevalence with the introduction of the cause upon which it chiefly depends.
The colon has a length of about 5 feet. Its greatest width, about 3 inches, is at its commencement, but it is everywhere much wider than the small intestine. Whereas the wall of the small intestine is smooth externally, the wall of the colon is sacculated. Three muscular bands constrict it longitudinally; circular bands at intervals of about 1 inch or 1½ inch throw it into pouches. It ascends on the right side, lying far back against the body-wall, to which it is bound by peritoneum, which in this part of its course covers only its anterior surface. Having touched the under side of the liver, it loops forwards and to the left side, crossing the middle line just above the umbilicus. On the extreme left side it touches the spleen, getting very near to the back of the abdominal cavity. It then descends on the left side, again bound to the body-wall by peritoneum, although not so closely as on the right side, until it reaches the inner lip of the crest of the hip-bone. From here onwards the fold of peritoneum which attaches it allows it a free movement. This portion of the large intestine, the sigmoid flexure, may even fall over into the right groin. Lastly it curls backwards into the pelvis, as the rectum.