134 MICROSCOPIC ANATOMY OF THE TEETH 



fixation of the tissue, the length of time during which the 

 preparation is exposed to the staining reaction, and the 

 composition and concentration of the stain, while many com- 

 pound stains are unreliable. These membranes are, however, 

 clearly visible in unstained preparations of marsupial enamel 

 organs and in those treated with hsematoxylin, and require 

 no differential stain to make them apparent. 



2. Calcification 



With the exception of horny teeth which are hardened by 

 the cornification of the stratum corneum of the epithelium, 

 all teeth are calcified, or impregnated with lime salts. The 

 organic matter in which they are first laid down is per- 

 meated with the salts in different degrees, producing 

 structures of sufficient hardness to resist the severe strain 

 to which they are subjected, the calcification of enamel 

 being so complete that its original structure is considerably 

 veiled or lost, and it is the densest and hardest of all the 

 animal tissues. 



The formation of enamel and the mode of deposition of 

 the lime salts have long been among the most difficult 

 problems in the histology of the teeth. 



Many contradictory theories have been held on the 

 subject, and while much fresh light has been thrown upon 

 it in late years by the investigations of Rose, Von Ebner, 

 Leon Williams, and others, there, still remain many diffi- 

 culties to be overcome in arriving at a clear understanding 

 with regard to it. 



Owing to the complicated courses of the enamel prisms, 

 the difficulty of obtaining sections in which the calcified 

 portion of the tissue is in normal relations with the formative 

 cells, and the great care necessary to avoid distortion of the 

 preparations and the consequent deceptive appearances 

 produced, there are few tissues that present more difficulties 

 to the investigator. 



The subject of the calcification of enamel is so intimately 

 bound up with certain chemical and physical phenomena, 

 that a brief statement of these is necessary before proceed- 

 ing to a more detailed consideration of the subject, that it 

 may be made clear, at all events in some degree, what part 



