THE LIVING WORLD
Unit Six. Animal Life
25. The Path of Food Through the Animal Body
25.4. The Mouth and Teeth
Specializations of the digestive systems in different kinds of vertebrates reflect differences in the way these animals live. Fishes have a large pharynx with gill slits, whereas airbreathing vertebrates have a greatly reduced pharynx. Many vertebrates have teeth (discussed below), and chewing (mastication) breaks up food into small particles and mixes it with fluid secretions. Birds, which lack teeth, break up food in their two-chambered stomachs. The first chamber, the proventriculus (figure 25.7), produces digestive enzymes, which are passed along with the food into the second chamber, the gizzard. The gizzard contains small pebbles ingested by the bird, which are churned together with the food by muscular action. This churning grinds up the seeds and other hard plant material into smaller chunks that can be digested more easily in the intestine.
Figure 25.7. The digestive tract of birds.
In birds, food enters the mouth and is stored in the crop. Because birds lack teeth, they swallow gritty objects or pebbles, which lodge in the gizzard, to help pulverize food. Digestive enzymes produced in the proventriculus are churned up with the food and gritty objects in the gizzard before passing into the intestine.
Recall from chapter 20 that while reptiles and fish have homodont dentition (teeth that are all the same), most mammals have heterodont dentition, teeth of different specialized types. Up to four different types of teeth are seen: incisors, which are chisel-shaped and used for nipping and biting; “canines,” which are sharp, pointed teeth used for tearing food; and premolars (bicuspids) and molars, which usually have flattened, ridged surfaces for grinding and crushing food. The front teeth in the upper and lower jaws of mammals are incisors. On each side of the incisors are the canines. Behind the canines are premolars and then molars.
This general pattern of heterodont dentition is modified in different mammals depending on their diet (figure 25.8). For example, in carnivorous mammals the canines are prominent, and the premolars and molars are more bladelike, with sharp edges adapted for cutting and shearing. Carnivores often tear off pieces of their prey but have little need to chew them, because digestive enzymes can act directly on animal cells. (Recall how a cat or dog gulps down its food.) By contrast, grass-eating herbivores, such as cows and horses, must pulverize the cellulose cell walls of plant tissue before digesting it. In these mammals, the incisors can be well-developed and are used to cut grass and other plants. The canines are reduced or absent, and the premolars and molars are large, flat teeth with complex ridges well suited for grinding.
Figure 25.8. Diagram of heterodont dentition.
Different mammals have specific variations of heterodont dentition, depending on whether the mammal is an herbivore, carnivore, or omnivore. In this carnivore, the canines are prominent, and the premolars and molars are pointed—adaptations for tearing and ripping food. In herbivores, some of the incisors are large, the canines are reduced or absent, and the premolars and molars are flattened—adaptations for nipping and grinding vegetation.
Humans are omnivores, and human teeth are adapted for eating both plant and animal food. Viewed simply, humans are carnivores in the front of the mouth and herbivores in the back. Children have only 20 teeth, but these deciduous teeth are lost during childhood and are replaced by 32 adult teeth. The third molars are the wisdom teeth, which usually grow in during the late teens or early twenties, when a person is assumed to have gained a little “wisdom.”
As you can see in figure 25.9, the tooth is a living organ, composed of connective tissue, nerves, and blood vessels, held in place by cementum, a bonelike substance that anchors the tooth in the jaw. The interior of the tooth contains connective tissue called pulp that extends into the root canals and contains nerves and blood vessels. A layer of calcified tissue called dentin surrounds the pulp cavity. The portion of the tooth that projects above the gums is called the crown and is covered with an extremely hard, nonliving substance called enamel. Enamel protects the tooth against abrasion and acids that are produced by bacteria living in the mouth. Cavities form when bacterial acids break down the enamel, allowing bacteria to infect the inner tissues of the tooth.
Figure 25.9. Human teeth.
Each vertebrate tooth is alive, with a central pulp containing nerves and blood vessels. The actual chewing surface is a hard enamel layered over the softer dentin, which forms the body of the tooth.
Inside the mouth, the tongue mixes food with a mucous solution, saliva. In humans, three pairs of salivary glands secrete saliva into the mouth through ducts in the mouth’s mucosal lining. Saliva moistens and lubricates the food so that it is easier to swallow and does not abrade the tissue it passes on its way through the esophagus. Saliva also contains the hydrolytic enzyme salivary amylase, which initiates the breakdown of the polysaccharide starch into the disaccharide maltose. This digestion is usually minimal in humans, however, because most people don’t chew their food for very long.
The secretions of the salivary glands are controlled by the nervous system, which in humans maintains a constant flow of about half a milliliter of saliva per minute when the mouth is empty of food. This continuous secretion keeps the mouth moist. The presence of food in the mouth triggers an increased rate of saliva secretion, as taste-sensitive neurons in the mouth send impulses to the brain, which responds by stimulating the salivary glands. The most potent stimuli are acidic solutions; lemon juice, for example, can increase the rate of salivation eightfold. The sight, sound, or smell of food can stimulate salivation markedly in dogs, but in humans, these stimuli are much less effective than is thinking or talking about food.
When food is ready to be swallowed, the tongue moves it to the back of the mouth. In mammals, the process of swallowing begins when the soft palate elevates, pushing against the back wall of the pharynx (figure 25.10). Elevation of the soft palate seals off the nasal cavity and prevents food from entering it 1. Pressure against the pharynx stimulates neurons within its walls, which send impulses to the swallowing center in the brain. In response, muscles are stimulated to contract and raise the larynx (voice box). This pushes the glottis, the opening from the larynx into the trachea (windpipe), against a flap of tissue called the epiglottis 2. These actions keep food out of the respiratory tract, directing it instead into the esophagus 3.
Figure 25.10. The human pharynx, palate, and larynx.
Key Learning Outcome 25.4. In many vertebrates, ingested food is fragmented through the tearing or grinding action of specialized teeth. In birds, this is accomplished through the grinding action of pebbles in the gizzard. Food mixed with saliva is swallowed and enters the esophagus.