

HISTOLOGICAL METHODS. 157 



owing to the clumsy and inconvenient method hitherto 

 adopted. This has simply consisted in placing the tissue 

 on tinfoil, or in a platinum capsule, and setting it in a 

 freezing mixture. It is very difficult and tedious to freeze 

 any large piece of tissue in this way, and when it is removed 

 from the freezing mixture, for the purpose of section, it 

 begins to thaw, and thereby gives rise to so much incon- 

 venience, and to results so unsatisfactory, that the method 

 of freezing has been almost neglected. The method is now, 

 however, rendered simple and satisfactory with the aid of 

 the freezing microtome (Fig. 58). It is convenient to study 

 its mode of employment in 302. 



METHODS OF SOFTENING THE TISSUES. 



283. Removal of Calcareous Matter. Chromic and 

 nitric acid both harden albuminous tissues, but soften those 

 which owe their hardness to the presence of calcareous 

 salts. Bone and tooth may be softened in the following 

 fluid : 



Chromic acid, i gramme. 



Water 2ooCC. Dissolve and add 



Commercial nitric acid, 2CC. 



This chromic and nitric fluid gives far better results than 

 dilute nitric or hydrochloric acid used alone, for while the 

 nitric acid more especially removes the calcareous matter, 

 the chromic acid especially hardens the bone protoplasts. 

 This fluid produces excellent results. For details, see 15. 

 Small pieces of bone may also be softened in a 

 ^ per cent solution of chromic acid, or in a solution of 

 picric acid, kept saturated by the presence of superabundant 

 crystals. The latter is preferable. This method is useful 

 for foetal bone ( 119). 



284. Softening of Connective Substances. Dilute 

 alcohol (rectified spirit * i part, water 2 parts) is of much 



* Ranvier recommends alcohol at 36 of Carder ; this is very 

 nearly the strength of ordinary rectified spirit, the sp. gr. of which is 

 0-838. 



