206 PRACTICAL HISTOLOGY. 



can be tested by attempting to pass a fine needle 

 through it the tooth is placed in spirit, which 

 should be changed day after day until it ceases to 

 become much colored by the excess of picric acid. 

 All that remains to be done is to cut the tooth in 

 halves vertically, and embedding one of the halves 

 in a hard wax-mass, cut thin slices from the artificial 

 section, stain these with logwood, and mount them 

 in glycerine. 



Preparation 5. Study of the teeth "in situ." 

 Still more instructive preparations are obtained by 

 softening a portion of the lower jaw with the teeth 

 in situ, and making sections through the whole struc- 

 ture. It is best to take the jaw of a small animal 

 a rat, for instance. The flesh having been cleared 

 away, the softening is effected with picric acid in 

 the way above described ; 'and then, after due immer- 

 sion in spirit, the piece is imbedded, cut, stained with 

 logwood, and mounted in glycerine as before. 



Besides showing the teeth and the way in which 

 they are inserted into the lower jaw, the structure 

 of this bone is itself well demonstrated. At the 

 lower part the constantly growing incisor, which 

 extends in the rat below the molars to the back part 

 of the jaw, exhibits the large elongated odontoblasts 

 of a developing tooth, with their well-marked denti- 

 nal processes (fibres of Lent), which in some parts 

 project like harp-strings across a small space which 

 intervenes between the cells and the dentinal sub- 

 stance. It will be remarked, also, that in these teeth 

 the most newly-formed layer of dentine becomes, 

 especially near its junction with the older parts, very 

 intensely stained by the logwood. This is the case 

 with all teeth which are still in process of develop- 

 ment. Carmine has not the same action. 



Preparation 6. Development of the teeth. 

 For the study of the development of the teeth sec- 

 tions are made of the jaws of embryos and young 

 animals. Perhaps the most convenient to choose are 

 newborn rats, since sections of their jaws exhibit not 



