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The digestive system, then, may be considered to present the following parts:
I. The alimentary canal, or digestive tube.
II. Special organs, found in the wall of this canal.
III. Accessory glands, placed external to the wall of the tube.
FIG. 873.-DIAGRAM OF THE GENERAL ARRANGEMENT OF THE DIGESTIVE SYSTEM.
The processus vermiformis is seen hanging down from the cæcum. The transverse colon is not represented, in order that the duodenum and pancreas, which lie behind it, may be seen.
The greater part of the digestive system is found in the abdomen, and hence, in this section, the abdominal cavity, together with its lining membrane the peritoneum, falls to be described.
I. Alimentary Canal.-The alimentary canal, taken as a whole, measures, when lly extended, about 30 feet (9 metres) in length, and consists (Fig. 873) of the
following parts in order: mouth, pharynx, œsophagus, stomach, small and large intestines. The term tubus digestorius is applied to the whole of the canal below the lower end of the pharynx. The mouth cavity is the first division of the tube. It is separated from the nasal cavities above by the palate, and opens posteriorly into the pharynx. This latter is an expanded portion of the canal lying posterior to the mouth, nasal cavity and larynx, the mouth opening into it through the isthmus of the fauces, the nasal cavity through the choane (O.T. posterior nares); whilst lower down, immediately below the base of the tongue, the aperture of the larynx is found in its anterior wall. Opposite the lower border of the larynx, the pharynx is continued into the oesophagus, a long and comparatively straight portion of the digestive tube, passing through the neck and thorax to the abdomen, which it reaches by piercing the diaphragm. Immediately after entering the abdomen the tube expands into a pear-shaped dilated chamber, the stomach. This is followed by over 20 feet of small intestine, the junction of the two being marked by a constriction, the pylorus. The small intestine presents three more or less arbitrary divisions-namely, (a) the duodenum, a part about 10 inches in length, curved somewhat like a horse-shoe, and closely united to the posterior abdominal wall; (b) the jejunum, which includes the upper two-fifths, and (c) the ileum, the lower three-fifths of the small intestine beyond the duodenum. The jejunum and ileum are connected to the posterior abdominal wall by the mesentery, a fan-shaped fold of connective tissue covered by the peritoneum, or lining membrane of the abdominal cavity.
The terminal part of the ileum opens into the side of the large intestine, a few inches (2) from the blind commencement of the latter. There is thus formed at the beginning of the great intestine a cul-de-sac, called the cæcum, in connexion with which there is a small worm-shaped diverticulum, the vermiform
The orifice through which the ileum opens into the large intestine is guarded by the valve of the colon (O.T. ileo-cæcal valve), which prevents the return of its contents from the large into the small bowel. After the cæcum comes the ascending colon, which runs up on the right side of the abdomen. This is succeeded, in order, by the transverse colon, crossing from right to left, the descending colon, running down on the left side of the abdomen, and the iliac colon, lying in the left iliac fossa. Beyond this are the pelvic colon, which lies in part or entirely within the pelvis minor (O.T. true pelvis), the rectum, and the anal canal.
The rectum lies within the pelvis minor, and the anal canal, the terminal part of the intestine, is a short channel passing between the muscles which form the pelvic floor, to open on the surface at the anal orifice.
The B.N.A. term colon sigmoideum includes the portion named above as pelvic colon, and the term colon descendens includes the descending and iliac colon.
II. Special Organs found in the Wall of the Tube.-In the mouth are found the teeth, gums, tongue, and behind them, in the pharynx, are the palatine tonsils. The teeth, 32 in number in the adult, are portions of the mucous membrane of the mouth and of the subjacent tissue, calcified on the surface, and specially formed for mastication, that is, the division and triturition of the food which take place in the mouth before the bolus, as the resulting mass is termed, can be swallowed. They are rooted in the jaws and are partly surrounded by the gums.
The tongue is a muscular organ, useful alike in mastication, deglutition, and speech. It is covered with epithelium, which in places is modified so as to form taste corpuscles, which are the end organs of the gustatory sense.
The roof of the mouth is formed by the palate, which separates the mouth from the nose. It consists of a bony part in front called the hard palate, and a movable sheet, called the soft palate, behind.
The palatine tonsils are two large masses of lymph tissue, found one on each side of the wall of the pharynx, just posterior to the mouth. They form the most prominent portions of an almost complete ring of lymph tissue placed around the circumference of the tube at this level.
III. Accessory Digestive Glands. The largest of these is the liver (hepar) which occupies the upper and right portion of the abdominal cavity, immediately below the diaphragm, and its secretion-the bile-is conveyed into the duodenum by the bile duct (ductus choledochus). The pancreas, next in size, lies across the front of the vertebral column, with its right end or head resting in the concavity of the duodenum, into which its secretion flows through the pancreatic duct. The salivary glands consist mainly of three large paired glands, parotid, submaxillary, and sublingual (glandula parotis, submaxillaris, and sublingualis), and their ducts, which convey the saliva, open into the mouth. The saliva is a mechanical lubricant, which facilitates swallowing and the movements of the tongue in speaking and masticating, and also plays an important part in the chemical processes of digestion.
1. Parts.-Rima oris, vestibulum oris, cavum oris proprium.
2. Boundaries.-1. Labia oris-their structure.
2. Bucca-their structure.
3. Palatum (palatum durum, palatum molle)-arrangement and structure. 4. Isthmus faucium.
3. Structures found in the Mouth.-Gingivæ, gums.
4. Glandulæ oris.-Buccal and salivary glands.
The mouth is the upper expanded portion which forms the first division of the alimentary canal. It lies between the maxillæ and mandible, bounded externally by the lips and the cheeks, and roofed in by the palate. It contains the teeth and greater part of the tongue; and the ducts of the salivary glands open
into it. The cavity is divisible into two portions, the vestibule and the cavity proper of the mouth. These are separated from one another by the alveolar ridges, gums, and teeth of the maxillæ and mandible. The cavity of the mouth narrows at the back to a slight constriction, marked by a vertical fold on each side, called the arcus glosso-palatinus (O.T. anterior pillar of the fauces), and between them the cavity of the mouth is continuous with that of the pharynx.
Rima Oris.-The aperture of the mouth is bounded above and below by the corresponding lips, which, by their junction at the sides, form the labial commissures. In a state of rest, with the lips in apposition, the rima appears as a slightly curved line, corresponding in length to the interval between the first premolar teeth, and in level to a line drawn across just below the It also shows the two palatine arches, and the pharyngo- middle of the upper incisor crowns. nasal isthmus, through which the naso-pharynx, above, The shape of the rima varies with communicates with the oral portion of the pharynx, every movement of the lips, from the resting linear form, curved like the conventional bow, to a circular or oval shape when the mouth is widely open. or the "pursed-up" condition produced by the contraction of the orbicularis oris muscle.
Vestibulum Oris.-The vestibule of the mouth lies immediately internal to the
aperture of the mouth. It is that portion of the cavity which occupies the interval between the lips and cheeks externally, and the teeth and gums internally.
In the normal resting condition, when the mouth is closed and the lips and the teeth are in contact, its cavity is practically obliterated by the meeting of its walls, and it becomes merely a slit-like interval, with a narrow roof and floor formed by the reflection of the mucous membrane from the deep surface of the lips and cheeks to the corresponding gum. This reflection is interrupted in the median plane by a small but prominent fold of the mucous membrane, the frenulum, which connects the back of each lip to the front of the gum. The upper frenulum is the better developed, and is readily brought into view by everting the lip. The frenulum of the lower lip is not always present.
On the outer wall of the vestibule, opposite the crown of the second upper molar, upon a variably developed eminence, is placed the small opening of the duct of the parotid gland, which conveys the saliva from the parotid gland to the mouth.
When the teeth are in contact the vestibule communicates with the cavity of the mouth only through the small and irregular spaces left between the opposing teeth, and posteriorly, on either side, by a wider but variable aperture between the last molars and the ramus of the mandible.
Advantage is some
times taken of the pres
Deep part of submaxillary gland pulled back
Submaxillary duct (Wharton's)
Cut edge of mucous membrane
Hyoid branch of lingual artery
ence of this aperture for the introduction into the cavity of the mouth of liquid food in certain cases trismus, anchylosis, etc.-in which the jaws are rigidly closed.
On the outer wall of the vestibule, the anterior border of the masseter can be distinctly felt with the finger, when the muscle is thrown into a state of contraction. Still further back, the front of the coronoid process, bearing the lower part of the insertion of the temporal muscle, can also be made out. The spheno-mandibular ligament, which corresponds to, and is felt along with, the anterior border of the internal pterygoid muscle, is distinguishable as a pliant ridge when the finger is carried from the front of the coronoid process behind the last molar tooth into the cavity of the mouth.
In addition to the duct of the parotid, the ducts of numerous small glands which are embedded in the lips and cheeks open into the vestibule.
Under normal conditions, as pointed out above, the lips and cheeks lie against the teeth and gums, obliterating the cavity of the vestibule, and helping, with the aid of the tongue, to keep the food between the grinding surfaces of the molar teeth during mastication. In facial palsy, however, owing to the paralysis of their muscles, and particularly of the buccinator muscle, the lips and cheeks fall away from the dental arches, and allow the food to pass out from between the teeth and to accumulate in the vestibule.
Cavum Oris Proprium.-The cavity proper of the mouth is the space situated within the dental arches, extending backwards to the glosso-palatine arches (O.T. anterior pillars of the fauces). Its boundaries consist of a roof, a floor, and a margin, formed by the teeth and gums. The roof is formed by the hard palate and the anterior portion of the soft palate, while the floor is formed by the anterior part of the tongue in the middle, and on each side by the reflection of the mucous membrane from the side of the tongue to the mandible.
On each side of the tongue, and in front of it, when it is at rest, there is only a slit or sulcus between the tongue and the gums, into which the ducts of the submaxillary and sublingual glands open.
If, however, the tongue is raised, there is exposed a limited space to which the term sublingual space is more usually applied (Fig. 876).
The term "floor of the mouth," or sublingual region, is frequently applied to the muscular and other structures, especially the mylo-hyoid muscles, which fill in the interval between the two halves of the body of the mandible. These structures, with the hyoid bone, form the basis upon which the tongue and the mucous membrane of the sublingual space are supported, and they extend from the symphysis menti, in front, to the body of the hyoid bone, behind.
The sublingual region (Fig. 876) is covered by the mucous membrane between the deep surface of the gum and the inferior aspect of the tongue. When the tip of the tongue is raised the membrane forms in the median plane a prominent fold,
the frenulum linguæ, stretching from the floor of the mouth to the inferior surface of the tongue. On each side of the frenulum, near its junction with the floor, there can be readily made out a prominent soft papilla, the caruncula sublingualis, on which the opening of the duct of the submaxillary gland (O.T. Wharton's duct) may be seen (Fig. 876). Running laterally and posteriorly, on each side, from this, Layer of muscle cut and occupying the greater part of
Anterior gland of
to show the gland
Duct of the submax.
FIG. 876.-OPEN MOUTH WITH TONGUE RAISED, AND THE
the floor of the mouth, there is a well-marked ridge, plica sublingualis, due to the projection of the underlying sublingual gland. Most of the ducts of this gland open near the crest of the ridge on each side. There is also another fold, called the plica fimbriata, medial to each of the others, on the inferior surface of the tongue.
When the mouth is closed, and respiration is carried on through the nose, the cavum oris is reduced to a slit-like space, and practically obliterated by the tongue coming in contact with the palate above, and with the gums and teeth laterally and in front. When the mouth is slightly open and the teeth nearly in contact, the tongue becomes somewhat concave or grooved along the median plane, and leaves a channel-like space between it and the palate, while it remains in contact with the roof and gums laterally. By depressing the hyoid bone together with the root of the tongue, the cavum oris can be increased to a considerable size even when the teeth are in contact. Finally, by the simultaneous descent of the mandible and hyoid bone with the tongue, and the ascent of the soft palate, the cavity is increased to its greatest dimensions (Fig. 874).
The sublingual gland of the left side has been laid bare by the removal of the mucous membrane; to expose the anterior lingual gland of the right side a thin layer of muscle, in addition to the mucous membrane, has been removed. A branch of the lingual nerve is seen running on the medial aspect of the gland. The profunda vein also is faintly indicated on this side.
Labia Oris.-The lips are the two movable folds, covered superficially by skin, and on their deep surface by mucous membrane, which surround the rima oris. The meeting of the lips at each side constitutes the commissure, and bounds the angle of the mouth (angulus oris). Laterally, they are prolonged into the cheeks, with which they are continuous. The junction of the lips and cheek is marked on the surface by the sulcus naso-labialis, which passes downwards and laterally from the margin of the nose towards the angle of the mouth, while the sulcus mento-labialis separates the lower lip from the chin. The upper lip presents on its superficial surface a well-marked vertical groove, the philtrum, bounded by two distinct ridges descending from the columella nasi (Fig. 876); inferiorly the groove