EXAMINATION OP THE HARD TISSUES. 673 



enamel, and its tendency to chip off, considerable difficulty is 

 experienced in obtaining sections of teeth for examination. Sec- 

 tions having been obtained, they may be moistened with water, 

 turpentine, and Canada balsam, and examined by transmitted 

 light with low powers. A tooth that has been macerated in strong 

 acid for several days can be very readily cut in any direction. 



The dentinal tubules are microscopic canals, pursuing a waving 

 and anastomosing course through the whole thickness of the 

 dentine, from the wall of the pulp-cavity to the cement and 

 enamel. According to Kblliker, each canal has a special wall, 

 rather less in thickness than its diameter, which can only be ob- 

 served in transverse sections, as a narrow yellowish ring sur- 

 rounding the cavity ; in longitudinal sections, on the other hand, 

 it is almost entirely invisible. These tubules may be isolated 

 from each other by long maceration in acid, and subsequent 

 soaking in dilute caustic soda or potash. 



The enamel consists of hexagonal or pentagonal prisms, one 

 extremity of which is attached to Nasmyth's membrane, the 

 other to the dentine. These prisms are very closely united, 

 having no intervening substance between them. 



The development of the teeth may be studied in the lower 

 animals, or in human embryos at different ages. 



Nails. — By maceration in water the nail may be separated from 

 the skin and thin sections made by means of a sharp knife. 



A nail consists of two layers, the upper or horny one forming 

 the true nail, and marked with ridges on its lower surface, while 

 the under soft one is continuous with the rete mucosum of the 

 skin, from which it differs by the cells being elongated and ar- 

 ranged perpendicularly. The horny layer is composed of flat- 

 tened epidertnic cells aggregated into laminae. The addition of 

 caustic potash or soda in solution causes the nuclei to be de- 

 veloped. 



Hair. — A white hair is best adapted for demonstrating the 

 intimate structure of the shaft and other parts. For this purpose, 

 solutions of soda and potash and strong sulphuric and acetic 

 acids are found to be the most useful reagents. 



The cortex, which constitutes a large part of the shaft, presents 

 upon its surface a great number of longitudinal striae or inter- 

 rupted dark lines and dots. Upon treating it with sulphuric 

 acid at a gentle heat, as recommended by Kblliker, it is first 

 changed into plates and fibres, varying in length and breadth, 

 and afterwards converted into elliptical or spindle-shaped cells, 

 which become flattened and angular from pressure. These cells 

 contain elongated, dark-looking nuclei and pigment-granules, to 

 which the color of the hair is due. The addition of caustic pot- 

 ash or soda will isolate these pigment-granules, and sometimes 

 cause them to exhibit molecular motion. The presence of small 

 spaces containing air, and the unequal refraction of light by dif- 



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