THE DIGESTIVE SYSTEM 



455 



next (proceeding posteriorly), then two 

 premolars, used in grinding food, and fi- 

 nally tlnee molars which are the heavy 

 grinders. The last molars, the wisdom teeth, 

 may appear late in life or not at all. 



Teeth that are so perfectly formed in 

 lower vertebrates and primitive man seem 

 to have difficulty withstanding the inroads 

 of civilization. It has long been known that 

 the white man has notoriously bad teeth 

 while his primitive brother usually has per- 

 fect teeth throughout his lifetime. In recent 

 years much research has been done to solve 

 this perplexing and distressing problem. A 

 high carbohydrate diet has been thought to 

 be responsible for tooth decay, but natives 

 of many of the South Sea Islands live 

 almost exclusively on a starchy diet and 

 yet their teeth are unusually well pre- 

 served. However, when sugar (sucrose) 

 replaces starch in the diet, dental caries 

 ( decay ) appear. Along with this change to 

 sugar there is a marked increase in the 

 numbers of an acid-forming bacterium 

 {Lactobacillus acidophilus) in the mouth. 

 If sugar is withheld from the diet for some 

 time the bacteria disappear and there is no 

 further decay of the teeth. It is rather well 

 confirmed today that it is the sugar in the 

 diet that causes dental caries, and since 

 sugar occurs only in small quantities in 

 nature, it can be considered an unnatural 

 food for all animals including man. Teeth 

 are not designed to withstand the action of 

 acids produced by the bacteria, hence de- 

 cay is prevalent among sugar-eating people, 

 which includes much of the civilized world. 

 Fluorine in the water is known to be of 

 value in preserving teeth, probably owing 

 to the fact that in addition to the hardness 

 of the enamel it produces, it retards the 

 growth of bacteria in the mouth. 



Digestion in the mouth 



The only part of the saliva that has to do 

 with food breakdown is the enzyme ptyalin. 

 It acts directly on starches and converts 

 them to maltose, a disaccharide (double) 



sugar. This is a very active enzyme, for if 

 only a few drops of saliva are added to a 

 suspension of starch kept at body tempera- 

 ture, it will be converted to maltose in 

 about 20 minutes. This demonstrates one of 

 the most remarkable properties of an en- 

 zyme, for to accomplish the same conversion 

 in the laboratory would require drastic 

 treatment with powerful acids for many 

 hours. Ptyalin has the power to break the 

 large starch molecules into the much smaller 

 maltose molecules, leaving only one more 



paro1-Jd gland 



pdrol-id ducf 



("osta bud 

 sublingual gland 



submaxillary gland 



Fig. 17-4. The salivary glands secrete saliva when end 

 organs in the tongue and mouth cavity are stimu- 

 lated. The nerve pathway is shown here. 



step to the glucose stage when, as a sim- 

 ple sugar, it can be taken into the body 

 tissues and burned. The way most people 

 eat, ptyalin has only a momentary chance 

 to function before the food departs for the 

 stomach, although its action does continue 

 during swallowing and for a short time in 

 the stomach. 



The flow of saliva into the mouth is con- 

 trolled by nerves which respond to stimuli 

 coming not only from the taste buds on the 

 tongue but also from end organs in the 

 walls of the oral cavity that are sensitive to 

 the presence of food itself and to the me- 

 chanics of chewing. Impulses travel from 

 these end organs to the brain stem to be 



