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 eight con- 

 sists of: (1) two incisors, the chisellike cut- 

 ting teeth, in the front of the jaw; (2) one 

 canine, the blunt-pointed tearing tooth at 

 the side of the incisors; (3) two premolars, 

 the simple grinding teeth behind the ca- 

 nines; and (4) three molars, the complex 



Crn 



Bone 



Fig. 16-14. Structure of a human tooth, diagrammatic. 

 Enl, enamel; Den, dentine; Cm, cement; Crn, crown; 

 Nk, neck; Rt, root. 



