152 PRACTICAL HISTOLOGY. 



height. The cylinder is set upon the cloth, and the lens 

 (immersion) is screwed down through the aperture in the 

 elastic membrane, as represented in vertical section in C, 

 where a is the thick and e the thin glass plate of the moist 

 calico, g the cover-glass upon the object, h the glass cylinder, 

 and m the caoutchouc membrane in contact with the objec- 

 tive immersed in a drop of fluid on the cover-glass. Any 

 evaporation from the chamber is fully compensated by the 

 passage of fluid through the cloth by capillary attraction 

 below the margin of the cylinder. 



272. When it is necessary to preserve the living tissue 

 at a constant temperature above that of the surrounding 

 air, a hot stage (Fig. 49) must be had recourse to, and it 

 seems likely that the moist chamber last described (Fig. 53) 

 might be placed on a hot stage of suitable dimensions, and 

 the effect of heat upon tissue-growth thus observed under 

 high powers for a lengthened period. 



METHODS OF HARDENING THE TISSUES. 



Some tissues are too soft, others too hard, to permit of 

 their being cut or dissected ; it is therefore necessary to 

 alter their consistence. 



273. Drying and Boiling were formerly much resorted 

 to, but are now rarely employed. The former may be used 

 for tendon ( 104), when transverse sections are wanted 

 merely to show the relations of the various systems of 

 fibres ; moreover, it is sometimes advantageous to partially 

 dry a tissue that has been hardened in alcohol before at- 

 tempting to cut it. If, however, protoplasm and other soft 

 matters have not been previously hardened, they are gene- 

 rally spoilt by desiccation. Boiling is useful for hardening 

 the fibres of the crystalline lens ( 236), and also those 

 of muscle, when coarse dissections are to be made of the 

 latter. 



274. Chromic Acid is much employed as a hardening 

 agent. A dilute solution of it in water may be used alone 

 for hardening the lung, spleen, uterus, muscle, cornea, 



