The Digestive System Of Humans And Other Vertebrates
Although an examination of the structure of the human digestive tract reveals little in the way of general principles that could not as easily be seen in an earthworm, natural interest in our own species and ourselves prompts a more detailed examination of human anatomy and the different biological systems it contains.
The Oral Cavity
The first chamber of the digestive tract is, of course, the oral cavity. Located here are the teeth, which function in the mechanical breakup of food by both biting and chewing. Human teeth are of several different types, each adapted to a different function. In front are the chisel-shaped incisors, four in the upper jaw and four in the lower, which are used for biting. Then come the more pointed canine teeth, one on each side of each jaw, which are specialized for tearing food. Behind each canine are two premolars and three molars in adults; these have flattened, ridged surfaces, and function in grinding, pounding, and crushing food. A child’s first set of teeth does not include all of those mentioned here; the first (or milk) teeth are lost as the child gets older, and are replaced by the permanent teeth that have been growing in the gums.
The teeth of different species of vertebrates are specialized in a variety of ways and may be quite unlike human teeth in number, structure, arrangement, and function, as seen when studied under a compound light microscope. For example, when viewed under a compound light microscope, the teeth of snakes are very thin and sharp and usually curve backward; they function in capturing prey, but not in mechanical breakup, for snakes do not chew their food, but swallow it whole. The teeth of carnivorous mammals, such as cats and dogs, are more pointed than human teeth; the canines are long, and the premolars lack flat grinding surfaces, being more adapted to cutting and shearing. On the other hand, such herbivores as cows, horses, and deer have very large flat premolars and, molars with complex ridges and cusps as seen under a compound light microscope; the canines are often absent in such animals.
Notice that sharp pointed teeth, poorly adapted for chewing, seem to characterize meat eaters like snakes, dogs, and cats, whereas broad flat teeth, well adapted for chewing, seem to characterize vegetarians. How can this difference be explained? Plant cells, when examined under a microscope, are enclosed in a cellulose cell wall. Very few animals can digest cellulose; most of them must break up the cell walls of the plant they eat if the cell contents are to be exposed to the action of digestive enzymes. Animal cells like those in meat, when viewed under a microscope, do not have any such nondigestible armor and can be acted upon directly by digestive enzymes. Therefore chewing is not as necessary for carnivores as for herbivores. You have doubtless seen how dogs gulp down their food, while cows and horses spend much time chewing. But carnivores have problems of their own. They must capture and kill their prey, and for this, sharp teeth capable of piercing, cutting, and tearing are well adapted. Humans, being omnivores, have teeth that belong, functionally and structurally, somewhere between the extremes of specialization attained by the teeth of carnivores and herbivores.
The oral cavity has other functions besides those associated with the teeth. Here food is tasted and smelled, activities of great importance in food selection, and the food is mixed with saliva secreted by several sets of salivary glands. The saliva dissolves some of the food and acts as a lubricant, facilitating passage through the next portions of the digestive tract. Human saliva contains a starch-digesting enzyme, which initiates the process of enzymatic hydrolysis, as seen under a microscope. It also contains an antimicrobial agent, together with a special enzyme that facilitates the agent’s entry into microbial cells, as seen under a microscope; these substances help prevent infection by potentially harmful microbes, which are regularly introduced into the mouth.
The muscular tongue manipulates the food during chewing and forms it into a mass, called a bolus, in preparation for swallowing. It then pushes the bolus backward through a cavity called the pharynx (throat) and into the esophagus. The pharynx functions also as part of the respiratory passageway; the air and food passages cross here, in fact. Swallowing, therefore, involves a complex set of reflexes that close off the opening into the nasal passages and trachea or windpipe, thereby forcing the food to move into the esophagus. As you know, these reflexes occasionally fail to occur in proper sequence, and the food, entering the wrong passageway, makes you choke.
