400 Our Surroundings 



Each tooth consists of three parts. The cro'tun, covered with 

 hard white enamel, is the visible part of the tooth, above the gum. 

 The tooth extends through the gum and is rooted in the jaw. 

 The part of the tooth encased in the gum is 

 called the neck. The third part, the root, is 

 surrounded by a bony substance called cement, 

 and is imbedded in the jawbone. The tooth 

 is composed largely of an ivory-like material 

 called dentine. The interior is filled with a 

 substance called pulp, which contains the nerves 

 CROSS SECTION OF A an d blood vessels that enter it through small 

 Showi J^enamei, hole s at the ends of the roots. 



dentine and pulp. Salivary Glands. There are three pairs 



of salivary glands : the sublingual, lying under the sides of the 

 tongue; the submaxillary, lying just beneath the angles of the 

 lower jaw at each side, and the parotid, lying in the cheeks just 

 in front of the ears. The parotid glands are the seat of the 

 disease called the mumps. 



These glands secrete saliva, which moistens all food taken into 

 the mouth, and by means of an enzyme or digestive ferment, called 

 ptyalin, digests to some extent the starchy part of food. 



Pharynx and Esophagus. The pharynx is the pear-shaped 

 cavity lying between the mouth and the gullet, or esophagus. 

 It is a little over four inches in length in an adult. Its 

 walls have seven openings into other parts of the body. The two 

 that concern digestion open into the mouth and the esophagus. 

 Of the other five, one opens into each nostril, one into each ear, 

 and one into the trachea, or windpipe. This last opening has a 

 movable cover which closes when swallowing occurs, to keep food 

 out of the windpipe. A person would choke if food slipped into 

 the trachea. Just in front of the pharynx on each side there is 

 a fleshy tissue called the tonsil. 



The esophagus, the next part of the alimentary canal, 

 leads to the stomach. The esophagus is open only during the 

 passage of food. When not filled out by food as it is forced 

 downward, the esophagus lies flat, its walls pressed or crumpled 

 together. 



