156 PREPARATION AND MOUNTING 



lions where the sections are required to show the position of 

 the different vessels, &c. ; but, as before stated, is very valu- 

 able for all soft substances. As an instance of this, it may 

 "be mentioned, that it is frequently used in cutting sections 

 of sponges ; but as these are often very full of spicula, 

 it is much better to press the sponge flat until dry, and then 

 cut off thin shavings with a very sharp knife ; these shav- 

 ings will expand when placed in water. After this they 

 may be laid betwixt two flat surfaces and dried, when they 

 may be mounted as other dry objects, or, when desirable, in 

 balsam. 



Valentin's knife is very much used in taking sections of 

 skin, which are afterwards treated with potash solution, 

 acids, &c., to bring out in the best way the different por- 

 tions. Dr. Lister's mode, however, of getting these is thus 

 given in the Microscopic Journal : " But I afterwards 

 found that much better sections could be obtained from 

 dried specimens. A portion of shaved scalp being placed 

 between two thin slips of deal, a piece of string is tied 

 round them so as to exercise a slight degree of compression ; 

 the preparation is now laid aside for twenty -four hours, 

 when it is found to be dried to an almost horny condition. 

 It then adheres firmly by its lower surface to one of the 

 slips, and thus it can be held securely, while extremely thin 

 and equable sections are cut with great facility in any plane 

 that may be desired. These sections, when moistened with 

 a drop of water and treated with acetic acid, are as well 

 suited for the investigation of the muscular tissue as if they 

 had not been dried." 



There are many who almost confine their attention to 

 polarized light and its beautiful effects. Such would not 

 deem these efforts to aid the student in cutting sections 

 complete, without some notice of those which are taken 

 from various crystals, in order to display that curious and 

 beautiful phenomenon, Hie rings with a cross. The arrange- 

 ment of these is somewhat changed by the crystal which 

 affords the section ; but nitrate of potash gives two sets of 



