306 HOW TO WORK 



4. After having remained in this fluid for three or four days, it 

 will be found that the portions of soft pulp have regained the volume 

 they occupied when fresh. They have swollen out again even in the 

 strongest glycerine. 



5. I have found that in many cases, when it is desired to study 

 the arrangement of the nerves, it is necessary to harden the pulp by 

 immersion in a glycerine solution, made by adding to an ounce of the 

 glycerine solution of the acetic acid, two or three drops of a strong 

 solution of chromic acid, p. 297. The fragments may remain in this 

 solution for three or four days, and then be transferred to the acetic 

 acid solution, in which they may be preserved for years with all the 

 soft parts perfect. 



6. The specimens are now ready for examination. Thin sections 

 are cut with a knife from the fractured surfaces of the dentine, includ- 

 ing a portion of the soft pulp. The knife should be strong, but 

 sharp. In practice I have found the double-edged scalpels made for 

 me by Messrs. Weiss and Son, of the Strand, answer exceedingly well 

 for this purpose, nor will the edge of the knife be destroyed so soon 

 as would be supposed. 



7. The minute fragments of sections thus obtained are placed 

 upon a slide and immersed in a drop of pure strong glycerine, in 

 which they may be allowed to soak for an hour or more, and then 

 examined by a low power (an inch). The best pieces are to be 

 selected by the aid of a fine needle, and removed to a drop of glyce- 

 rine containing two drops of acetic acid to the ounce, and placed 

 upon a clean slide. The thin glass cover is then carefully applied, 

 and the specimen may be examined with higher powers. 



8. If it is desired to retain the specimen, the excess of glycerine 

 fluid is absorbed by small pieces of blotting-paper, and the glass 

 cover cemented to the slide by carefully painting a narrow ring of 

 Bell's microscope cement round it. When this first thin layer is dry, 

 the brush may be carried round a second time, and after the lapse 

 of a few days, more may be applied. Mounted in this way the speci- 

 men will retain its character for years. 



Hard tissues, like bone, dentine, and enamel, become somewhat 

 softened by prolonged maceration in glycerine, and if a few drops 

 of acetic acid are added, the softening process may be earned 

 still further, and yet without the calcareous matter being dissolved 

 out to any perceptible extent. If desired, of course the calcareous 

 matter may be in part or entirely removed by increasing the strength 

 of the acid fluid in which the preparation is immersed. But, far 

 short of this, the hard, brittle texture is so altered, that thin sections 

 may be cut without any difficulty. Specimens prepared in this way 



