Organs of the Digestive Tract
As we study the organs of the digestive system, locate each in Figure 15-4. The digestive tract is a muscular tube extending through the body. It is composed of several parts: the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The digestive tract is sometimes called the alimentary tract, from the word aliment, meaning “food.” It is more commonly referred to as the gastrointestinal (GI) tract because of the major importance of the stomach and intestine in the process of digestion. The next section describes the structure and function of each digestive organ. These descriptions are followed by an overview of how the organs work together in the digestive process.
The mouth, also called the oral cavity, is where a substance begins its travels through the digestive tract (Fig. 15-5). The mouth has the following digestive functions:
* It receives food, a process called ingestion.
* It breaks food into small portions.
This is done mainly by the teeth in the process of chewing or mastication, but the tongue, cheeks, and lips are also used.
* It mixes the food with saliva, which is produced by the salivary glands and secreted into the mouth. Saliva lubricates the food and has a digestive enzyme called salivary amylase, which begins starch digestion. The salivary glands will be described with the other accessory organs.
* It moves proper amounts of food toward the throat to be swallowed, a process called deglutition.
The tongue, a muscular organ that projects into the mouth, is used as an aid in chewing and swallowing, and is one of the principal organs of speech. The tongue has a number of special organs on its surface, called taste buds, which can differentiate taste sensations (bitter, sweet, sour, or salty).
Figure 15-4 The digestive system.
Figure 15-5 The mouth. The teeth and tonsils are visible in this view.
The oral cavity also contains the teeth (see Fig. 15-5). A child between 2 and 6 years of age has 20 teeth, known as the baby teeth or deciduous teeth. (The word deciduous means “falling off at a certain time,” such as the leaves that fall off the trees in autumn.) A complete set of adult permanent teeth numbers 32. The cutting teeth, or incisors, occupy the anterior part of the oral cavity. The cuspids, commonly called the canines or eyeteeth, are lateral to the incisors. They are pointed teeth with deep roots that are used for more forceful gripping and tearing of food. The molars , the larger grinding teeth, are posterior. There are two premolars and three molars. In an adult, each quadrant (quarter) of the mouth, moving from anterior to posterior, has two incisors, one cuspid and five molars.
The first eight deciduous (baby) teeth to appear through the gums are the incisors. Later, the cuspids and molars appear. Usually, the 20 baby teeth all have appeared by the time a child has reached the age of 2 to 3 years. During the first 2 years, the permanent teeth develop within the upper jaw (maxilla) and lower jaw (mandible) from buds that are present at birth. The first permanent teeth to appear are the four 6-year molars, which come in before any baby teeth are lost. Because decay and infection of deciduous molars may spread to new, permanent teeth, deciduous teeth need proper care. As a child grows, the jawbones grow, making space for additional teeth. After the 6-year molars have appeared, the baby incisors loosen and are replaced by permanent incisors. Next, the baby canines (cuspids) are replaced by permanent canines, and finally, the baby molars are replaced by the permanent bicuspids (premolars). At this point, the larger jawbones are ready for the appearance of the 12- year, or second, permanent molar teeth. During or after the late teens, the third molars, or so-called wisdom teeth, may appear. In some cases, the jaw is not large enough for these teeth, or there are other abnormalities, so that the third molars may not erupt or may have to be removed. Figure 15-6 shows the parts of a molar. The main substance of the tooth is dentin, a calcified substance harder than bone. Within the tooth is a soft pulp containing blood vessels and nerves. The tooth’s crown projects above the gum, the gingiva, and is covered with enamel, the hardest substance in the body. The roots of the tooth, below the gum line in a bony socket, are covered with a rigid connective tissue (cementum) that helps to hold the tooth in place. Each root has a canal containing extensions of the pulp.
The pharynx is commonly referred to as the throat (see Fig. 15-5). The oral part of the pharynx, the oropharynx, is visible when you look into an open mouth and depress the tongue. The palatine tonsils may be seen at either side of the oropharynx. The pharynx also extends upward to the nasal cavity, where it is referred to as the nasopharynx and downward to the larynx, where it is called the laryngeal pharynx. The soft palate is tissue that forms the posterior roof of the oral cavity. From it hangs a soft, fleshy, Vshaped mass called the uvula.
Figure 15-6 A molar tooth.
In swallowing, the tongue pushes a bolus of food, a small portion of chewed food mixed with saliva, into the pharynx. Once the food reaches the pharynx, swallowing occurs rapidly by an involuntary reflex action. At the same time, the soft palate and uvula are raised to prevent food and liquid from entering the nasal cavity, and the tongue is raised to seal the back of the oral cavity. The entrance of the trachea is guarded during swallowing by a leaf-shaped cartilage, the epiglottis, which covers the opening of the larynx. The swallowed food is then moved into the esophagus.
The esophagus is a muscular tube about 25 cm. (10 inches) long. In the esophagus, food is lubricated with mucus and moved by peristalsis into the stomach. No additional digestion occurs in the esophagus. Before joining the stomach, the esophagus must pass through the diaphragm. It travels through a space in the diaphragm called the esophageal hiatus. If there is a weakness in the diaphragm at this point, a portion of the stomach or other abdominal organ may protrude through the space, a condition called hiatal hernia.