200 PREPARATION OF SPECIMENS IN VISCID MEDIA. 



applied, which would be destructive to specimens mounted in 

 water. Again, these media have a preservative action, so that if 

 the tissues be permeated by them soon after death, further 

 changes are prevented. They have, moreover, the effect of ren- 

 dering the tissues more transparent, and enabling their components 

 to be more readily distinguished. It has been objected that these 

 viscid media are unsuitable, as causing the tissues to shrink, and 

 soft cells to collapse, by the exosmose of their fluid contents ; but 

 in reply it is stated by Dr. Beale, that though such shrinkage is 

 the immediate effect of the use of a viscid medium of consider- 

 able density, tissues left in it for a few days recover their 

 original dimensions. "I have preparations," he says (Op. cit. 

 p. 294), "from creatures of every class. The smallest Animalcules, 

 tissues of Entozoa, Polypes, Starfishes, Mollusks, Insects, Crus- 

 tacea, Infusoria, various Vegetable Tissues, microscopic Fungi and 

 Alga3 of the most minute and delicate structure, as well as the 

 most delicate parts of the higher Vegetable tissues, may all be 

 preserved in these viscid media ; so also may be preserved the 

 slowest and the most rapidly-growing, the hardest and the softest 

 Morbid growths, as well as Embryonic structures at every period 

 of development, even when in the softest state. All that is 

 required is, that the strength of the fluid should he increased 

 very gradually, until the whole tissue is thoroughly penetrated by 

 the strongest that can be obtained.'" " Minute dissections can be car- 

 ried on in the seviscid media with greater facility and certainty than 

 in more limpid fluids. I can readily detach the most minute parts 

 of tissues, separate the different structures in one texture without 

 tearing or destroying them, unravel convoluted tubes, and perform 

 with ease a great variety of minute operations, which it would be 

 impossible to effect with any of the ordinary methods of dissection. 

 With care in regulating the temperature, I can soften textures thus 

 preserved in syrup to the precise extent required for further 

 minute dissection ; and even very hard textures (such as Bone and 

 Teeth) may thus be softened, so that by gradually increased 

 pressure and careful manipulation exceedingly thin layers can be 

 obtained, without the relation of the anatomical elements to each 

 other bein°: much altered, and without any of the tissues being 

 destroyed." (Op. cit. p. 205). Dr. Beale recommends that any 

 Re-agents used in making preparations of this kind, should them- 

 selves be dissolved in Glycerine. 



Section 2. Mounting of Objects. 



147. The Microscopist not merely desires to prepare Objects for 

 examination, but, where possible, to preserve them in such a 

 manner that they may be inspected at any future time. This may 

 be so effectually accomplished in regard to many substances, that 

 they undergo no kind of change, however long they may be re- 

 tained ; and even delicate structures whose composition renders 



