300 ANIMAL BIOCHEMISTRY 



where animals have died of lead poisoning. The composition of bone 

 can be influenced by a number of factors. Lactating animals in high 

 production maintain normal amounts of calcium and phosphorus in 

 milk at the expense of their skeletons, even when the diet is supple- 

 mented with these minerals. Periods of lowered production are 

 needed to allow skeletal recovery. 



The ossification process is hampered in rickets (page 357), osteo- 

 malacia, and similar bone diseases. These conditions may be initiated 

 by calcium and phosphorus starvation, by unbalanced Ca : P ratio, by 

 absence of sunlight or vitamin D (page 355), or by pathological 

 conditions. 



Teeth 



The teeth are bony structures consisting of mineral matter embedded 

 in an organic matrix. The principal constituents of teeth are enamel, 

 cement, and dentine. Cement represents the layer covering the root 

 of the tooth. It is a hard material similar to bone, containing about 

 70 per cent of mineral matter and an organic matrix consisting largely 

 of collagen. The surface of the tooth is composed of an extremely 

 hard material called enamel, which contains from 96 to 98 per cent 

 of mineral matter with a small amount of organic material consisting 

 largely of keratin. The main body of the tooth, which lies underneath 

 the enamel and next to the pulp, is composed of dentine. This struc- 

 ture contains about 77 per cent of mineral matter and an organic 

 matrix containing collagen. 



Enamel, although similar to bone in chemical composition, seems 

 to contain more calcium and phosphorus and less magnesium and 

 water. Also, enamel is characterized by the presence of a hydroxyl 

 group, which does not appear to be present in bone. Enamel has 

 been described as inert, once deposited. However, recent studies with 

 tagged elements have shown that radioactive phosphorus injected 

 into an animal soon appears in the tooth and that about 10 per cent 

 of it is found in the enamel layer. Dentine and cement occupy an 

 intermediate position between enamel and bone, so far as chemical 

 composition is concerned. Fluorine appears to be an essential consti- 

 tuent which provides a defense against dental caries. 



There are two schools of thought regarding the cause and pre- 

 vention of dental caries. One group emphasizes the importance of 

 proper diet, involving quantity and quality of calcium and phosphorus 

 and other dietary essentials such as fluorine and vitamins A, C, and D. 

 The other group believes that dental hygiene plays the major role. 



