248 ANIMAL BIOLOGY 



to get usually in his first year, are but twenty in number. 

 These are later replaced by the permanent teeth. Each 

 tooth is set into the jaw bone by one or more roots. 

 There is a cavity in each tooth filled with pulp which 

 contains blood vessels and nerves and is sensitive to pain. 

 The body of the tooth is composed of a bony substance 

 called dentine, and the exposed part is covered by a very 

 hard layer, the enamel. 



In animals below man the teeth vary much in number, 

 size and shape. In the carnivores they are fitted for 

 tearing flesh, while in the herbivores they are adapted for 

 grinding food. The incisors are well developed in the 

 rodents or gnawers, while in the elephant the upper 

 incisors are modified into the enormous tusks which afford 

 us our ivory. In fishes, amphibians and reptiles the 

 teeth are generally conical and fitted for seizing prey, 

 which is their primitive function. These animals as a 

 rule do not masticate their food but swallow it whole. 

 Teeth are subject to decay which is usually caused by the 

 lodgment of particles of food that undergo decomposition. 

 When the decay reaches the pulp cavity we are generally 

 reminded of the fact and are compelled to seek the services 

 of the dentist. Keeping the teeth clean by frequent use 

 of the tooth brush prevents the beginning of trouble. 



The Salivary Glands. Three pairs of salivary glands 

 pour their secretion, the saliva, into the mouth cavity. 

 The saliva is an alkaline fluid containing a considerable 

 amount of mucus and a ferment, ptyalin, which con- 

 verts starch into sugar. Chewing food not only divides 

 it into smaller particles, but it mixes it with saliva which 

 thus has a better opportunity to act upon the starchy 

 constituents. 



The Stomach and Gastric Digestion. When we swallow 

 our food it passes through a long tube, the esophagus, into 



