516 



VERTEBRATE LIFE AND ORGANIZATION 



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Bone of ja-W 



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Figure 26.1. Diagram of a section through a human molar tooth. (Modified after 

 Maxiniow and Bloom.) 



malian tooth (Fig. 26.1) consists of a crown projecting above the gum 

 and one or more roots embedded in sockets in the jaws. The crown is 

 covered by a layer ol enamel. Enamel is the hardest substance in the 

 body and consists almost entirely of crystals of calcium salts. Calcium, 

 phosphate and fluoride are important constituents of enamel and all 

 must be present in the diet in suitable amounts for proper tooth de- 

 velopment and maintenance. The rest of the tooth is composed of dentin, 

 a substance very similar to bone. In the center of the tooth is a pulp 

 cavity containing blood vessels and nerves. A layer of cement covers 

 much of the root and holds the tooth firmly in place in the jaw. 



The teeth of most vertebrates are cone-shaped structures used pri- 

 marily for seizing and holding the prey. In mammals, the teeth are 

 differentiated into several types that are used not only for seizing food 

 but also for its mechanical breakdown. Mammalian teeth, unlike those 

 of lower vertebrates, are not continually replaced. Man, for example, 

 first has a set of deciduous, or milk teeth— two incisors, one canine and 

 two premolars on each side of each jaw. These are later replaced by 

 permanent teeth; in addition, three molars develop on each side of each 

 jaw behind the premolars. The molars last throughout life and are not 

 replaced. 



Once the food is in the mouth, a fish easily manipulates and 

 swallows it, for the flow of water aids in carrying it back into the 

 pharynx. Oral glands and a tongue are poorly developed in fishes. The 

 evolution of these structures accompanied the transition from water to 

 land and they became more elaborate in the higher tetrapods. In addi- 

 tion to a liberal sprinkling of simple glands in the lining of the mouth 

 cavity, mammals have evolved three pairs of conspicuous salivary glands 

 that are connected to the mouth by ducts. The location of the parotid, 

 submaxillary and sublingual glands of man is shown in Figure 26.2. 

 Originally oral glands simply secreted a mucous and watery fluid to 



