9 8 



HUMAN BIOLOGY 



Crown 



Enamel 



Dentine 



having become semi-liquid, and the proteids having been 

 partly digested by the gastric juice, an acid secretion from 

 the small glands in the stomach walls. 



(4) The small intestine, a narrow tube more than twenty 

 feet long, where the fats are acted upon for the first time, 

 and where the starches and proteids are also acted upon, 

 and where, after about ten hours, the digestion of the 

 three classes of foods is completed by pancreatic juice 

 from the pancreas, and bile from the liver. 



(5) The large intestine, about five feet long, where the 

 last remnant of nutriment is absorbed, and the indigestible 

 materials in the food are gathered together (Exp. 9). 



The Teeth. - The main body 

 of the tooth consists of bone- 

 like dentine, or ivory. Hard, 

 shining enamel protects the 

 crown, or visible portion. The 

 part of the tooth beneath the 

 gum is called the neck, and the 

 part in the bony socket is called 

 the root. The enamel ends just 

 beneath the gum, where it is 

 overlapped by cement of the 

 root. There is a pulp cavity 

 in every tooth (Fig. 91); it 

 contains pulp made up of con- 

 nective tissue, with nerves and 

 blood vessels which enter at the 

 tip of the root (Exp. 6). 



The temporary set of teeth is 

 completed at about two years of age and consists of twenty 

 teeth. The teeth cannot grow as the jaw grows, and soon 

 a larger and permanent set starts to growing deeper in the 



cavity 



Heck 



r.oot 



Bone 



Cement or crust.i petrosa 



Alveolar periosteum or root- membrane 



FIG. 91. CANINE TOOTH CUT 

 LENGTHWISE. 



