Mouth
The mouth, which is the first portion of the alimentary canal, is adapted to receive food and prepare it for digestion (fig. 1-53). The mouth mechanically reduces the size of solid particles and mixes them with saliva. This process is called mastication. Saliva, produced by the salivary gland, moistens food making it easier to chew. Saliva also lubricates the food mass to aid swallowing. The tongue assists with both mastication and swallowing.

Pharynx
The pharynx (covered earlier in "The Respiratory System") is the passageway between the mouth and the esophagus and is shared with the respiratory tract (fig. 1-53). The epiglottis is a cartilaginous flap that

TYPES LOCATIONS SENSES
Superficial receptors At or near surface of body Touch, pressure, heat, cold, and pain
Deep receptors In muscles, tendons, and joints Sense of position and movement
Internal receptors In the internal organs and blood vessel walls
Usually none (except hunger, nausea, pain from stimuli such as chemicals (e.g., aspirin) and distension (e.g., stomach expansion from gas))

Table 1-4.-Types of Receptors, Their Location, and Affected Sense

Figure 1-53.-The digestive system.

Table 1-5.-Principal Digestive Juices

closes the opening to the larynx when food is being swallowed down the pharynx. Food is deflected away from the trachea to prevent particle aspiration (inhalation).

Esophagus
The esophagus is a muscular tube about 25 cm (10 inches) long (fig. 1-53). It is the passageway between the pharynx and the stomach. By means of peristalsis, food is pushed along this tube to the stomach. When peristalsis is reversed, vomiting occurs.

Stomach
The stomach acts as an initial storehouse for swallowed material and helps in the chemical breakdown of food substances. The stomach is a saccular enlargement of the gastrointestinal tube and lies in the left upper quadrant of the abdomen (fig. 1-53). It connects the lower end of the esophagus with the first portion of the small intestine (the duodenum). The stomach is divided into the cardiac, fundic, body, and pyloric regions (fig. 1-54). At each end of the stomach, muscular rings (or sphincters) form valves to close off the stomach. The sphincters prevent the stomach's contents from escaping in either direction while food substances are being mixed by peristaltic muscular contractions of the stomach wall. The sphincter at the esophageal end is the cardiac sphincter; at the duodenal end it is the pyloric sphincter.

The chemical breakdown of food in the stomach is accomplished through the production of digestive juices (enzymes) by small (gastric) glands in the wall of the stomach. The principal digestive enzymes produced by the gastric glands are hydrochloric acid and pepsinogen. Hydrochloric acid activates pepsin from pepsinogen, kills bacteria that enter the stomach, inhibits the digestive action of amylase, and helps regulate the opening and closing of the pyloric sphincter. Pepsin is a protein-splitting enzyme capable of beginning the digestion of nearly all types of dietary protein.

Most food absorption takes place in the small intestine. In general, food is not absorbed in the stomach. An exception is alcohol, which is absorbed directly through the stomach wall. It is for this reason that intoxication occurs quickly when alcohol is taken on an empty stomach.