192 ANIMAL PHYSIOLOGY. [LESSON 54. 



LESSON LIV. 



THE STRUCTURE OF THE TEETH, CONTINUED. 



823. Examined in transverse section, the prisms of enamel are 

 well seen throughout their course ; they are distinguished by a series 



FIG 309 of transverse lme s at regular intervals (Fig. 



309), which appear to divide the tissue into 

 small squares, or dice. When the prisms are 

 isolated (as we have seen), the same appearance 

 appears to be caused by the transverse lines on 

 their surface. On the other hand, it has been 



supposed, and with probability, that these 



section of transverse lines indicate the several stages of 

 growth of the tissue, and that, although division 

 of it in the direction indicated by the lines is impossible, the evi- 

 dence of demarcation defining its original stages of growth, is ren- 

 dered permanent. If the surface of enamel be destroyed, the facets 

 of the prisms are lost, as already indicated. 



824. The dentine or ivory of a recent tooth is of a yellowish- 

 white color, and transparent ; in a dried tooth, air enters the tubuli, 

 and imparts to it a silky or satiny appearance. The dentine forms 

 the walls of a case, in which the pulp is received r so that the tubes of 

 this tissue have uniform contact with the vessels of the pulp. At the 

 lower part of the fang of a tooth there is a small aperture, through 

 which the vessels pass to supply the pulp, and through which also the 

 nerve, to be divided into a delicate looped plexus, is transmitted. 



825. The dentine consists of minute canals, or tubuli, which 

 commence by open mouths on the inner wall of the pulp cavity, and 

 extend through the whole substance of the dentine to the cement and 

 enamel. Each tube has a wall, less than its own diameter in sub- 

 stance, which can easily be seen in transverse sections ; in longitudi- 

 nal sections it is almost invisible. In life these canals contain a 

 clear fluid, but in a dead tooth the tubuli become filled with air ; and 

 as air in small cavities always appears black under the microscope, 

 these tubuli appear as so many small, delicate black lines ; but if the 

 tooth be prepared in Canada balsam, the air is driven out of the tu- 

 buli by the heat employed in making the preparation, and the tur- 

 pentine of the balsam enters them; in this case they appear as deli- 

 cate tubes. 



826. The direction of the tubuli in the dentine is rarely straight, 



