The mouth (oral cavity) contains the tongue and teeth. The epithelium of the mucosa lining the cheeks and tongue is able to resist the wear and tear in-volved in chewing food, and in most regions it is a stratified, squamous (non-keratinised) epithelium. This epithelial surface is moistened by secretions from small serous and mucous glands in the submucosal layer and from salivary glands (Fig. 12.3) which drain their saliva into the oral cavity. Physical breakdown of food occurs in the mouth and it is mixed with salivary gland secretions which begin to break down large carbohydrate molecules.

There are three, paired salivary glands, the parotid, submandibular and sublingual glands. Each gland is invested by a connective tissue capsule, and connective tissue forms trabeculae which penetrate the glands and provide support for the blood vessels and nerves supplying the glands and for the ducts draining them. Each gland has secretory cells arranged as compound, tubuloacinar, exocrine glands (Chapter 3). The secretory acini drain into small ducts (lined by a simple cuboidal epithelium) which join together in a branching system; the larger ducts are lined by columnar epithelial cells, which may be stratified. The ducts drain the saliva into the oral cavity.

Two types of secretory cell, serous and mucous cells, are present in salivary gland acini (Fig. 12.3) and, respectively, they produce a watery secretion containing proteins which function as enzymes and a viscous mucus in which large carbohydrate complexes are major components. In addition, myoepithelial cells wrap around the acini and their contractions help to expel the secretions. The secretions assist the process of digestion by moistening and lubricating the food, and by providing enzymes. The main enzyme is amylase, which breaks down large carbohydrate molecules. Saliva is also responsible for lubricating and cleansing the oral cavity. It contains bactericidal substances such as lysozyme secreted by serous cells and may contain immunoglobulin A secreted by plasma cells.

The tongue is covered by a stratified, non-keratinised, squamous epithelium which on the undersurface is similar to the epithelium lining the mouth. The upper surface of the tongue is divided by a ‘V’-shaped groove into an anterior (two-thirds) and a posterior region developed from different parts of the embryo (see Mitchell B, Sharma R. Embryology: An Illustrated Colour Text. Elsevier: 2004). The stratified squamous epithelium on the upper surface is studded by prominent projections. In the posterior region, many of the projections are due to aggregations of lymphocytes deep to the epithelium. In addition, three distinct types of projection known as papillae are also described, and all assist in macerating food and resisting abrasion:

Deep to the mucosa of the tongue there are numerous, small serous and mucous glands which secrete onto the surface of the epithelium. The major component of the inner part of the tongue is skeletal muscle (see Fig. 5.7). The bundles of muscle cells are arranged in a complex three-dimensional meshwork and coordinated contraction of the muscle cells is important in chewing, swallowing and in sound production.

Swallowed food and drink pass via the pharynx to the oesophagus (gullet). The structure and function of the epithelium lining the component parts of the pharynx are described in Chapter 11. Aggregations of lymphoid cells are a prominent feature of the walls of the pharynx and form tonsils (Fig. 12.5). The lymphoid cells are important components of the immune mechanisms defending the alimentary canal (and the respiratory tract).

The major part of the oesophagus is in the thorax, but a small segment lies in the abdominal cavity and is continuous with the stomach. The structures in the walls of the oesophagus conform to the general plan for the gastrointestinal tract (Fig. 12.2). In the thorax, the connective tissue of the adventitia binds the muscularis externa of the oesophagus to adjacent structures. The muscularis externa consists of an outer, approximately longitudinal layer and an inner circular layer. In the upper part of the muscularis externa, striated voluntary muscle (Fig. 12.6) is present, whereas in the lower region it is replaced by smooth involuntary muscle. In the middle region there is a transition between the two types of muscle. Coordinated contraction of muscle cells in the different regions of the oesophagus ensures that swallowed food and drink normally pass to the stomach. In some regions, the submucosal tissue of the oesophagus contains serous and mucous glands in addition to blood vessels, nerves and aggregations of lymphoid cells. The epithelium of the mucosa lining the oesophagus is stratified and squamous (Fig. 12.6) and is able to resist friction from swallowed food and chemical attack by swallowed drinks, e.g. alcohol.

The stomach is a dilated portion of the gastrointestinal tract. It is described as having four regions: the cardiac region, the fundus, the body and the pylorus (Fig. 12.1). The cardiac region is continuous with the lower end of the oesophagus. The fundus is the part of the stomach lying closest to the diaphragm and is a site, in humans, where gas collects. The body of the stomach is the largest part of the stomach and the pylorus connects the body of the stomach to the duodenum. The stomach produces 2–3 litres/day of gastric secretions (juices) and the main constituents are mucus, water, hydrochloric acid and enzymes able to digest carbohydrates, fats and proteins. The mixture of gastric juices and ingested food and drink is known as chyme. The structure of the stomach conforms to the general plan (Fig. 12.2) with some modifications which relate to its functions.