Mouth. -- The mouth is separated from the nose by the hard and soft palate, and communicates. It is bounded in front by the lips, and its sides by the cheeks. The space between the lips and teeth is called the vestibule. The mouth is lined by a mucous membrane, which is covered by numerous glands, some being mucous and some salivary. The mouth contains a double row of teeth, thirty-two in the aggregate, performing the first process in digestion, the mastication of food.

Tongue. -- The tongue is an oblong, flattened, muscular body, which varies in size and shape; it is the organ of taste, and also of importance in speech and mastication. Its posterior extremity or root is attached to a bone, called the hyoid, by yellow fibrous tissue. Its anterior extremity is called the tip; its intervening portion its body. The mucous covering of the tongue is very thick upon its upper surface, and very thin upon its under surface. Upon its upper surface are a number of projections, of various sizes and shapes, called papillae. The largest are eight or nine in number, called papillae maximae, and are situated at the posterior portion of the tongue, in two convergent lines. The smallest papillae are fine and pointed, and are found near the middle of the tongue, and are termed filiform. The tongue assists in the process of deglutition.

Palate. -- The palate separates the back portion of the nose from the mouth, and is divided into two parts. The hard palate, of a bony base, covered by mucous membrane, which is continuous with that of the mouth; the soft palate is the membranous separation between the back portion of the mouth and nose. From the middle the uvula projects, about three quarters of an inch in length; from each side of the uvula there are two divergent creswcdentic folds of mucous membrane, which are called lateral half-arches; the space between which constitutes the fauces.

Between the anterior and posterior arches of each side is the tonsil gland. The tonsils are about the size of an almond, and consist of a collection of large mucous follicles.

Salivary Glands. -- The salivary glands are of light pink color, and their secretion is of great service in mastication and digestion. There are three in number -- the parotid, submaxillary, and sublingual. The parotid is the largest; it lies on the side of the face in front of the ear, and beneath the skin. The submaxillary lies in a depression on the internal face of the lower jaw-bone. The sublingual is the smallest of the three; it is situated under the tongue.

Pharynx. -- The pharynx is a muscular and membranous sac, communicating with the mouth, nose, oesophagus, larynx, and the tube (Eustachian) leading to the ear. Its length is about five inches, although this varies by extension and contraction. Its uses are for deglutition, respiration, and modulation of the voice.

Oesophagus. -- This is the canal that conveys the food from the pharynx to the stomach. Its length is about nine or ten inches, and its diameter is not uniform, gradually increasing (as it descends). Its upper portion is the narrowest part of the alimentary canal; and hence foreign bodies which are too large to pass through the alimentary canal are generally arrested in the neck. It never contains air. Deglutition is performed by the contraction of the longitudinal fibres of the oesophagus, which shorten the passage, and by contraction of its circular fibres successively from above downward.

Stomach. -- The stomach is a conoidal sac, somewhat bent or curved, and situated below the breast-bone or in the epigastric region. The left extremity is much the larger, and terminates in a rounded sac; at the upper portions of this extremity is the cardiac orifice where the oesophagus is continued into the stomach, immediately below the diaphragm. The right extremity is continuous with the intestines, and its orifice is called the pyloric. The structure of the pylorus is much thicker than that of any other portion. The stomach is held in its position by the oesophagus and the duodenum, as well as by reflexions of the peritoneum. The upper and lower curvatures of the stomach are called the greater and lesser curvatures. Near the pyloric extremity of the stomach is a small dilatation called the antrum pylori. The dimensions of the stomach are variable, depending upon the mode of life. It has four coats; the peritoneal, muscular, cellular and mucous.

In the stomach the food receives the admixture of the gastric juice, which is the solvent agent of digestion. The fluids taken into the stomach are for the most part absorbed from it; the solids, with the exception of the insoluble parts, are by the action of the gastric juice reduced to a substance called chyme, which in general is grayish, semi-fluid, homogeneous, with a slightly acid taste and smell. The chyme is then poured into the duodenum through the pyloric orifice for the subsequent action of the intestines.

Intestines. -- The intestinal canal is from thirty to thirty-five feet in length, and is divided into large and small intestines. The small intestine is four-fifths of the length of the whole canal, reaching from the pylorus to the large intestine; it is cylindrical, and about one inch in diameter; there is a gradual diminution in calibre as it descends. Its coats are the same as those of the stomach. The mucous coat is very vascular, and its absorbents are very numerous. The glands are the crypts or follicles of Lieberkuhn, the glands of Peyer, the solitary glands, and Brunner's glands.

The small intestine is divided into duodenum, jejunam, and ileum.

The Duodenum commences at the pylorus, and is about twelve inches long. The common duct formed by the junction of the bile and gall ducts opens into it about four or five inches from the pylorus. The Jejunum (from jejunus, empty) constitutes the upper two-fifths of the small intestine, and the ileum the remaining three-fifths.

The large intestine reaches from the ileum to the anus, and is one-fifth in length of the whole canal; it differs much from the small intestine, and has a sacculated appearance. It likewise has four coats. It is divided into caecum, colon, and rectum.

The Caecum is a cul-de-sac or blind sac, and the commencement of the large intestine, and hence often called the caput coli. At the inferior portion is a worm-like process called the appendix vermiformis. On the side of the caecum is the ileo-caecal valve, an elliptical opening whereby the small intestine empties into the large.

The Colon is the largest portion of the large intestine; gradually diminishes in diameter until it terminates in the sigmoid or S-like flexure on the left side. It ascends on the right side, and forming an arch transversely, descends upon the left side. The Rectum is the terminating portion of the large intestine, and reaches from the sigmoid flexure to the anus. It is somewhat barrel-shaped, being larger in the middle than at either end.