MUSCULAR OB CONTRACTILE TISSUE. 



3. Necrosis occurs where the periosteum has been removed, or the 

 pulp has died. Here the tooth becomes rough, dark, and even 

 black, and at length falls out. 



4. Caries of the teeth, as in bone, is a gradual loss of substance. 

 Its cause is not well understood. Since it always commences on 

 the exterior, the fluids of the mouth are supposed to have an influ- 

 ence in producing it. There must, however, be a coincident putre- 

 factive decomposition of the organic elements of the tooth, which 

 becomes covered with infusoria and fungi. (Kolliker.) Indeed, 

 Ficinus thinks the latter are the principal cause of caries, since it 

 usually commences in the cracks and pits of the enamel, where un- 

 disturbed opportunity is given for these organisms to develop. 

 The discolored spots on the enamel first lose their salts, and then 

 break up into angular pieces. Next the dentine becomes soft, 

 yielding not more than ten per cent, of ash (Ficinus), and then de- 

 composed, and the dentinal tubuli become filled with the fluids pro- 

 ceeding from its decomposition ; and which, reaching the pulp, may 

 produce pain. Carious teeth contain an excess of carbonate of 

 lime. (Marchand.) 



5. In jaundice the teeth often become yellow, and in asphyxia, 

 red : the dentinal tubuli being penetrated by the coloring matter of 

 the bile, and the blood, respectively. In rickets, the teeth are not 

 affected (p. 334). The mucus upon the teeth always contains fungi ; 

 and, on accumulating, it hardens and constitutes the tartar of the 

 teeth. This consists of earthy phosphates, 79; mucus, 12.5; ptya- 

 line, 1, and organic matters soluble in hydrochloric acid, 7.5. (Berze- 



6. Finally, teeth are sometimes developed in abnormal situations, 

 as in ovarian cysts. 



CHAPTEK IX. 



MUSCULAR (CONTRACTILE) TISSUE, AND THE MUSCLES. 



SECTION I. 

 MUSCULAR OR CONTRACTILE TISSUE. 



MUSCULAR tissue has recently been regarded as presenting three 

 varieties: the elongated or fusiform contractile cell; the smooth 

 or non-striated muscular fibre, and the striated muscular fibre. 

 Kolliker has, however, shown that the elongated contractile cell 

 and the smooth muscular fibre are histologically identical. He, 

 therefore, includes both these under the name of "contractile (or 



