The Alimentary Canal is a tube about 28 feet long, which traverses almost the entire length of the axial part of the body. In man and all other vertebrates, it lies in relation to the ventral surface of the bodies of the vertebrae. It commences on the face at the orifice of the mouth, and terminates on the surface of the lower part of the trunk at the orifice of the anus. It is divided into a series of segments, or compartments, which communicate with each other, from above downwards, in the longitudinal axis of the canal. These compartments are named mouth, pharynx, oesophagus, stomach, small intestine (subdivided into duodenum, jejunum, and ileum), and large intestine (subdivided into caecum, colon, and rectum). The canal is lined by a mucous membrane, called the alimentary mucous membrane, which is continuous with the nasal mucous membrane, with the respiratory mucous membrane, and at the anal and oral orifices with the integument, Outside this mucous membrane is the submucous coat, and external to it is the muscular wall of the canal. By the contraction of the muscular wall the food is propelled along the canal from above downwards. Opening on the surface of the mucous membrane are the orifices of the ducts of numerous glands, the secretions of which, mingling with the food, act chemically on it, so as to render it soluble and capable of being absorbed.

The Mouth, Oral Cavity, or Buccal Cavity, is the dilated commencement of the alimentary canal, in which the food is masticated and mingled with the secretion of the salivary and mucous glands. It is situated in the face, and extends from the lips in front to the pharynx behind. It is bounded above by the hard and soft palate, with the uvula; below by the lower jaw, the mucous membrane of the floor of the mouth, and the tongue; on each side by the cheek; and in front by the lips, between which is the aperture of communication with the surface of the face. Behind it freely communicates with the pharynx through the isthmus faucium. The muscles situated in the lips, cheeks, floor of the mouth, tongue, and soft palate enter into the formation of the walls of the mouth.

The mouth is lined by a red-coloured mucous membrane, which becomes continuous posteriorly with that of the pharynx, and at the margins of the lips with the skin of the face. The mucous membrane covering the alveolar portions of the jaws, and surrounding the necks of the teeth, is called the gum. From the outer surface of each jaw it is reflected to the inner surface of the cheeks and lips. From the inner surface of the lower jaw the mucous membrane is reflected to the floor of the mouth, and a broad band, called froenum linguae, is prolonged to the middle line of the under surface of the tongue.

In its structure the mucous lining of the mouth consists of a stratified pavement epithelium, and a sub-epithelial fibro-vascular corium, possessing numerous vascular papillae. The mucous membrane of the gum is characterized by its density and toughness, due to the numerous strongly developed bundles of connective tissue in the corium, many of which are continued into the fibrous tissue of the periosteum, which covers the alveolar surface of the jaw. The free surface of the corium of the gum possesses numerous broad papillae, and is covered by a stratified pavement epithelium similar to that in the lips and cheeks. The mucous membrane of the hard palate is also tense and tough, though not so much so as the gum; and the fibrous fasciculi of its corium blend with the connective tissue of the subjacent periosteum. The mucous lining of the mouth is a sensitive membrane, and receives its nervous supply from the fifth cranial nerve.

The mucous membrane of the mouth is specially modified on the dorsum of the tongue, in the interval between the circumvallate papillae and the epiglottis, and in the substance of the tonsils, by the development of collections of lymphoid tissue in the sub-epithelial connective tissue.

Thé Tonsils are two almond-shaped bodies, situated, one on each side of the posterior orifice of the mouth, in the fossa between the anterior and posterior pillars of the soft palate. Their normal size is not bigger than a hazel nut, but they are very apt to enlarge, grow inwards across the posterior aperture of the mouth, and diminish the size of that opening. The free surface is marked by several rounded holes, leading into shallow pits or crypts, which may be either simple or branched, in the substance of the tonsil. The pits are lined by the epithelial covering of the mucous membrane, into which minute papiIlae project. In the subepithelial connective tissue of the walls of the crypts numerous follicles of lymphoid tissue are situated, and lymph cells are infiltrated in great numbers in the connective tissue between the follicles. Interspersed amidst the crypts are small racemose mucous glands. The tonsils are very vascular, and capillary blood-vessels are distributed in connection w ith the papillae, the lymphoid tissue, and the racemose glands The tonsillar veins form a plexus in relation to the attached surface of the tonsil.

Mucous and Salivary Glands of the Mouth.— The ducts of numerous glands, engaged in secreting mucus and saliva, open on the free surface of the mucous membrane of the mouth. Their secretion not only keeps the mouth moist, and aids therefore in articulation, but by mingling with the food assists in mastication, deglutition, and the digestive process. Each gland is characterized by being divided into small lobules, and by possessing a duct or ducts, which branch off in an arborescent manner in the substance of the gland, and finally end in the minute lobules. They all belong to the compound racemose group of glands.

The mucous glands of the mouth are situated beneath its mucous lining in the following localities :—(a) labial glands, in the upper and lower lips, but absent at the angles of the mouth; (6) buccal glands, scattered on the inner surface of the cheek from the lips to the opening of the parotid duct; (c) palatine glands, on the oral surface of the hard palate, in the uvula, on both surfaces of the soft palate, and in the tonsils; (sh) molar glands, close to the last lower molar tooth on each side; (e) lingual glands, extending backwards from the tip of the tongue along its margin, and also on its dorsum between the circumvallate papillae and epiglottis. The ducts of these mucous glands consist of a delicate membrane lined by a single layer of columnar epithelial cells. The terminal branches of the ducts which enter the lobules end in a series of saccular dilatations, the acini, alveoli, or gland-vesicles, which contain rounded or polygonal secreting cells. A collection of such vesicles forms a lobule. The lobules are bound together by intermediate connective tissue, in which the blood-vessels divide into a capillary network, that ramifies on the outer surface of the delicate membrane forming the wall of the glandvesicles.

The salivary glands of the mouth are the parotid, submaxillary, and sublingual glands. The parotid gland is the largest salivary gland, and occupies the parotid hollow between the lower jaw and the external ear. Its anterior border overlaps the masseter muscle, and the excretory duct emerges out of this border. A prolongation of gland substance, the socia parotidis, frequently accompanies the duct for a short distance. The excretory duct of the gland, called Stenson , s duct, passes forwards superficial to the masseter muscle, then pierces the cheek, and opens on its inner surface opposite the second upper molar tooth. The duct is between 2 and 3 inches long, and about the thickness of a crow-quill. The submaxillary gland is situated immediately below the lower jaw. The excretory duct of the gland, called Wharton's duct, runs forwards and opens on the floor of the mouth by the side of the fraenum linguae. The sublingual is the smallest of the salivary glands, and lies under the mucous membrane of the floor of the mouth, close to the fraenum linguae. It possesses from ten to twenty small excretory ducts, the ducts of Rivinus, some of which join Wharton’s duct, though the greater number open directly on the floor of the mouth near the fraenum linguae.

Structure.— The ducts of the salivary glands branch and terminate in the lobules,—each terminal duct ending in a series of saccular dilatations, the acini, alveoli, or glandvesicles, the wall of which, formed apparently of a membrana propria, is continuous with the simple membranous wall of the terminal duct. The terminal ducts are lined by a layer of squamous epithelium, and the gland-vesicles contain the secreting cells.

The blood-vessels are distributed in the interlobular connective tissue, and form a capillary network on the wall of the gland-ducts, and on the wall of the gland vesicles.

The Pharynx is an irregularly dilated canal, which forms a common passage, connecting the mouth with the oesophagus, and the nose with the larynx, so as to be subservient to the processes both of deglutition and respiration. Its position and connections have been described under the heading Anatomy.

The wall of the pharynx consists of three coats-—an external muscular and an internal mucous coat, and an intermediate fibrous membrane, which blends with the submucous coat. The muscular coat consists of three pairs of circularly arranged muscles, the constrictors of the pharynx; and of two pairs of longitudinally-arranged muscles, the stylo-pharyngei and plato-pharyngei, with occasionally a third pair, the salpingo-pharyngei. The constrictor muscles extend from the lateral wall to the middle line of the posterior wall of the pharynx, and are named from below upwards the inferior, middle, and superior constrictors; they lie on three different planes, so that the inferior constrictor overlaps the middle, and the middle the superior.

The mucous coat of the pharynx lines the canal, and is continuous through the several openings with the mucous membrane lining the Eustachian tubes, nose, mouth, larynx, and oesophagus.

The epithelium covering the mucous membrane of the nasal part of the pharynx is columnar and ciliated over a considerable surface, but elsewhere the pharyngeal epithelium is tesselated and stratified: and in the latter localities, vascular papillae project into the epithelial layers. Small racemose glands lie beneath the mucous membrane, which is pierced by their ducts to open on the surface (fig. 2); they are most numerous in the nasal part of the pharynx. Collections of lymphoid tissue are found in the sub-epithelial connective tissue, more especially in the nasal part of the pharynx, where it forms a mass, extending across the posterior and upper wall, between the openings of the two Eustachian tubes, which Luschka has called the pharyngeal tonsil. The arteries of the pharynx are derived from the external carotid or some of its branches. The motor, sensory, and sympathetic nerves unite to form the pharyngeal plexus situated behind the middle constrictor muscle.

The Soft Palate forms an inclined plane, which projects, downwards and backwards into the pharynx, from the posterior border of the hard palate. It is less dependent at the sides than in the mesial plane, where it forms an elongated' body, the uvula. Its anterior or oral surface is smooth, and gives origin on each side to a fold, which curves downwards to the side of the root of the tongue, to form the anterior pillars of the palate or fauces. Its posterior or pharyngeal surface, also smooth, gives origin on each side to a fold, which, springing from the base of the uvula, curves downwards and backwards to be lost in the side-walls of the pharynx; this pair of folds forms the posterior pillars of the palate or fauces. The soft palate is complex in structure, and consists of muscles, mucous membrane, glands, blood and lymph vessels, and nerves. The muscles of the soft palate are arranged in two groups, those which elevate and make it tense, and those which constrict the fauces.

The mucous membrane of the soft palate is continuous with that of the mouth and pharynx. The epithelium covering the anterior or oral surface is a stratified pavement epithelium. That on the posterior or pharyngeal surface is in infancy a lammated cylindrical and ciliated epithelium, with isolated areas of pavement epithelium, but in adults it is a laminated pavement epithelium. Numerous racemose mucous glands lie beneath the mucous membrane, but much more abundantly on the oral than on the pharyngeal aspect. Collections of lymphoid tissue, similar to those found in the tonsils, are also met with. The arteries are branches of the internal maxillary, facial, and ascending pharyngeal. The veins of the soft palate often assume a dilated character, and are continuous with the pharyngeal veins. Lymphatics are also distributed beneath the mucous membrane.

The (Esophagus, or Gullet, is an almost cylindrical tube, about 9 or 10 inches long, which transmits the food from the pharynx to the stomach. It commences in the neck opposite the body of the sixth cervical vertebra, where it is continuous with the pharynx. It passes down the lower part of the neck, traverses the cavity of the thorax, pierces the diaphragm at the oesophageal opening, enters the abdomen, and becomes continuous with the cardiac end of the stomach close to that opening.

Structure.— The wall of the oesophagus consists of three coats, named, from without inwards, muscular, submucous, and mucous coats.

The muscular or external -coat is divided into two layers, an external and an internal. The external layer is composed of fibres arranged longitudinally in the wall. The internal layer consists of fibres arranged in a series of rings around the tube, which lie sometimes horizontally, at others obliquely. The muscular coat in the upper fourth of the oesophagus is red, and its fibres are transversely striped; in the second fourth numerous non-striped fibres are mingled with the striped; whilst in the lower half the coat consists exclusively of non-striped fibres. By the contraction of the fibres of the muscular coat the food is propelled downwards into the stomach.

The submucous coat connects the muscular and mucous coats with each other. It consists of bundles of white fibrous tissue intermingled with elastic fibres, and the nerves and blood-vessels passing to the mucous coat ramify in it.

The mucous or internal coat lines the interior of the tube, and is continuous above with the mucous lining of the pharynx, and below with that of the stomach. When the oesophagus is empty it is thrown into longitudinal folds. Its free surface is covered by a thick layer of stratified squamous epithelium, which terminates abruptly at the cardiac orifice of the stomach in an irregular line. Projecting into the epithelium are multitudes of minute conical papillae. Opening on the surface of the membrane are the ducts of numerous small racemose glands similar to those in the pharynx (fig. 2). Collections of lymphoid tissue, forming solitary follicles, are also found in the mucous membrane. The deep surface of the mucous membrane consists of a layer of non-striped muscular tissue, the bundles of which run longitudinally; it forms the muscular layer of the mucous coat, or muscularis mτιcosce.

The oesophagus is supplied with blood by the inferior thyroid artery, the oesophageal branches of the thoracic aorta, and the ascending branch of the coronary artery of the stomach. The nerves are derived from the pneumo-gastrics, which form plexuses containing nerve-cells, not only in the muscular coat, but in the muscularis mucosae. A network of lymphatic vessels also occurs in both the mucous and submucous coats.

Abdominal Cavity and Peritoneum. —As the remaining portions of the alimentary canal are situated in the abdominal cavity, it will be advisable, before describing their anatomy, to give an account of the form and boundaries of that cavity, of its division into regions, and of the general arrangement «f the peritoneum, which constitutes its lining membrane.

The Abdominal Cavity, Abdomen, or Belly, is the largest of the three great cavities of the body. It occupies about the lower two-thirds of the trunk, and extends from the diaphragm above to the pelvic floor below. As its walls, except in the pelvic region, are chiefly formed of muscles and of fibrous membrane, they are much more distensible than those of the thorax, and permit considerable modifications to occur in the size of the viscera contained within the cavity. The abdomen is elongated in form; its vertical diameter is greater than either the transverse or the antero-posterior diameter. The superior boundary is formed by the concave vault of the diaphragm, and by the seven lower pairs of ribs and costal cartilages; in this boundary occur the opening through which the oesophagus passes into the abdomen, and also the apertures for the transmission of the great blood-vessels, the nerves, and the thoracic duct. The inferior boundary is formed by the levatores ani and coccygei muscles, and the pelvic fascia; in relation to this boundary are the termination of the rectum and anal orifice, the termination of the urethra, and in - the female that of the vagina also. The anterior boundary is formed above by the muscles of the anterior abdominal wall and the fascia transversalis; the linea alba occupies its middle fine, and about the middle of the linea alba is the umbilicus or navel; the anterior wall below is formed by the two pubic bones with the symphysis. The lateral walls, or flanks, arc formed above by the flat muscles of the abdominal wall and the fascia transversalis, and below on each side by the ilium and ischium with the muscles attached to them. The posterior wall is formed by the lumbar spine, sacrum, and coccyx, and by the muscles attached to these bones with their accompanying fasciae. The abdomen is primarily divided into the pelvis and abdomen proper. The pelvis is subdivided into the false pelvis, or the part above the pelvic brim, and true pelvis, or part below the pelvic brim.

The Peritoneum is the largest and most complicated serous membrane in the body. Like the other serous membranes, it not only lines the walls of the cavity in which it is situated, but gives a more or less complete investment to the contained viscera. It is arranged, therefore, so as to form a parietal and a visceral part, which are continuous with each other in the various regions where the part lining the wall is reflected as a covering upon the viscera. A space or cavity, called the peritoneal cavity, is inclosed between the parietal and visceral layers. This cavity, as in other serous membranes, is a closed or shut sac, without any communication externally, except in the female, where the two Fallopian tubes open into it.

Through these openings the mucous membrane lining the tubes becomes continuous with the serous membrane, and a communication is established between the lumen of each tube and the peritoneal cavity. That surface of both the parietal and visceral portion of the peritoneum which lies next the cavity is free, smooth, covered by an endothelium, and lubricated by a little serous fluid, which under some pathological conditions may be greatly increased in quantity, so as to cause abdominal dropsy. The moistening of the two free surfaces by the serum permits them to glide smoothly on each other, during the movements of the...