14 PREPARATION AND MOUNTING 



impossible in any oilier medium. All that is required is, 

 that the strength of the fluid should be increased very 

 gradually until the whole tissue is thoroughly penetrated by 

 the strongest that can be obtained ;" and " that thus very 

 hard textures may be softened, so that by gradually increas- 

 ing pressure and careful manipulation exceedingly thin 

 layers can be obtained, without the relation of the anatomi- 

 cal elements to each other being much altered, or any of the 

 tissues destroyed." He also takes occasion to observe, " that 

 tissues immersed in water are destroyed by even moderate 

 pressure ; but that in a viscid medium (such as glycerine or 

 syrup) the requisite pressure can be borne not only without 

 injury or impairment of the discrimination of their parts, 

 but with advantage to their detail." One very great advan- 

 tage which results from the use of glycerine for the prepara- 

 tion of textures is, that however they may swell in it after 

 prolonged immersion, a sufficient soaking in water will 

 always restore them to their normal condition. Another is, 

 that on account of its very high refractive power, it is 

 peculiarly fitted for the preparation of structures to be in- 

 vestigated by polarized light, with the same advantage as in 

 the preceding case, that they are still amenable to all other 

 modes of inquiry. 



The caustic alkalies potash, soda, and ammonia, are 

 solvents of all animal textures except chitine, and perhaps 

 bone. As in nearly all cases a softening action, with little 

 or no alteration of tissue, precedes the solvent action, these 

 agents, and especially the first two, have their uses. Under 

 their influence " a condition is induced very favourable to 

 the imbibition of water, which afterwards penetrates very 

 rapidly, so that cells swell up and burst." They may be 

 used either with or without heat, and more or less dilute. 

 There is one disadvantage attending their use, that objects 

 can with difficulty be preserved after soaking in them. 



Heat, applied either by the aid of hot water or steam, or 

 the sand-bath, or a bath of fusible metals, or of melted let:d, 

 is a very efficient means of softening horny substances, whale- 



