The Inner (Alimentary) Tube and Its Respiratory Derivatives 35 



other ingredients of the bile seem to be waste materials which are 

 passed off via the intestine. During embryonic life the liver is a seat of 

 formation of red blood-cells. In the adult certain cells in the liver re- 

 move from the blood some of its effete red blood-cells. Summarizing, 

 the liver's functions include digestion, storage of products of digestion, 

 rationing of sugar to the blood, excretion of waste via kidneys and in- 

 testine, and collaboration with the spleen and other organs in main- 

 taining the efficiency of the red-cell population of the blood. 



The pancreas is less versatile than the liver, but probably no less 

 important. Its several chemically different secretions, either acting di- 

 rectly or interacting with secretions of liver and intestine, digest food 

 substances of all sorts — proteins, starch, and fats. The insulin, se- 

 creted into the blood by cells of the islands of Langerhans, regulates the 

 metabolism of sugar. 



Teeth 



Structure of Teeth 



The alimentary mechanisms of invertebrates include a variety of 

 structures which are functionally analogous to the teeth of vertebrates. 

 But a series of calcareous teeth carried in movable jaws at the entrance 

 to the alimentary canal is a type of dental equipment peculiar to the 

 vertebrates. 



The calcareous teeth in vertebrates of all classes have the same gen- 

 eral structure, but with differences in detail. This structure may be 

 seen to advantage in one of the relatively large and well-differentiated 

 teeth of a mammal. Three regions of the tooth are distinguishable, a 

 crown protruding into the oral cavity, a root buried in a bony socket 

 or alveolus, and a narrower region, the neck, between root and crown 

 (Fig. 30). The internal structure is best seen in a longitudinal section of 

 the tooth (Fig. 31). The wall of the tooth, enclosing a central pulp- 

 cavity, consists mainly of a calcareous substance, the dentine or 

 "ivory." Dentine is composed of minute calcareous tubules which radi- 

 ate from the pulp-cavity toward the outer surface of the tooth. In its 

 course from inner to outer surface of the layer, each tubule branches 

 several times. The exceedingly narrow cavities of the tubules are called 

 canaliculi. The dentine consists mainly (about 75 per cent in man) of 

 mineral salts, chiefly calcium phosphate and calcium carbonate. About 

 25 per cent of it is organic material. Usually at no place on the tooth 

 does the dentine reach the outer surface, since the crown and neck are 

 covered by a layer of calcareous substance, the enamel, while the root 

 is covered by a bonelike cement. 



Enamel is the hardest substance in the vertebrate body, containing 

 (in man) only about 3.5 per cent of organic material. Its mineral con- 



