. Principles of modern biology. Biology. The Digestive Sysiem - 299 Fig. 16-13. Gastric glands, fundus region of stomach. 1, pit on mucous surface of stomach; 2, neck of gland; 3, deep end of gland; 4, cells that produce HCI; 5, enzyme-producing cells. (From The Living Body, by Best and Taylor. Holt, Rinehart and Winston, Inc.) The Teeth. The teeth of different verte- brates are adapted to the food habits of the individual species, but vertebrate teeth gen- erally conform to a common structural pat- tern. The crown, which is the part that pro- jects above the gum level (Fig. 16-14), is made up
. Principles of modern biology. Biology. The Digestive Sysiem - 299 Fig. 16-13. Gastric glands, fundus region of stomach. 1, pit on mucous surface of stomach; 2, neck of gland; 3, deep end of gland; 4, cells that produce HCI; 5, enzyme-producing cells. (From The Living Body, by Best and Taylor. Holt, Rinehart and Winston, Inc.) The Teeth. The teeth of different verte- brates are adapted to the food habits of the individual species, but vertebrate teeth gen- erally conform to a common structural pat- tern. The crown, which is the part that pro- jects above the gum level (Fig. 16-14), is made up of three layers: (1) the enamel, an external covering of exceedingly dense hard material; (2) the dentine, an intermediate layer of bonelike matter, not quite so hard as the enamel; and (3) the pulp, a soft tissue (in- cluding the nerves and blood vessels of the tooth) that fills the central pulp cavity. The part of a tooth that is encircled by the fleshy gum tissues is called the neck; and below the neck lies the root, which fits snugly into a socket, provided by the jawbone. In compo- sition the root resembles the crown, but the enamel is replaced by cement, a material that binds the dentine to the bone of the jaw. Vertebrate teeth arose among primitive fish, from scalelike structures called placoid scales (p. 669). Such scales are found in the modern sharks and other cartilaginous fishes, but not in the bony fishes. The sharks have many rows of teeth, which rim the mouth in the region where the skin folds inward into the oral cavity; and these teeth are replicas of the smaller placoid scales that cover the entire body surface. The shark's teeth dis- play the same structural layers as other ver- tebrate teeth, but in higher vertebrates the teeth have become modified in form to fit the food habits of the particular species. The permanent teeth of adult man are normally 32 in number; there are 8 teeth on each side of both the upper and lower jaws (Fig. 16-15). Each group of ei
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