82 HOW TO WORK 



desirable, upon any part of the glass before it is removed to the com- 

 pressorium. A very convenient form is employed by M. Quatre- 

 fages, in which it is possible to examine the object upon either 

 side. 



The compressorium has been arranged by Mr. Ross so that the 

 object may be placed between two pieces of thin glass, and either side 

 of it subjected to examination under very high powers. Mr. Beck, I 

 think, improved upon the plan adopted by Mr. Ross. 



150. Cutting Sections of Tissues which have been previously 

 dried. There are, however, many tissues of which sections cannot 

 be obtained in this simple manner. It is almost impossible to cut 

 sections of soft membranous textures perpendicular to the surface, 

 sufficiently thin for examination. In such cases, we may pin out the 

 texture upon a board when perfectly fresh, and expose it to the 

 atmosphere, or over sulphuric acid under a bell jar, pi. XX, fig. 129, 

 until it is quite dry. Thin sections may then be cut very easily, and 

 upon being moistened with water they will resume their recent 

 appearance. The very delicate nervous tissue of the retina may be 

 cut into very thin sections by drying the eye after it has been cut 

 open, and pinned out flat on a board. The vitreous humour is not 

 to be entirely removed, as it protects the retina and dries up with it. 

 Very thin sections of the skin of various animals, certain vegetable 

 tissues, and of many other textures may be obtained by this 

 process. 



151. Hardening the Tissue. Some textures require different 

 treatment in order to render them sufficiently hard to enable us to 

 cut thin sections. Boiling in water is sometimes useful for this pur- 

 pose. Some tissues may be hardened by being soaked in alcohol, or 

 chromic acid, or in syrup, while not a few require special modes of 

 treatment, which are applicable to them alone. See part III, where 

 the use of various solutions for hardening is described. 



Cutting thin Sections of Hard Tissues. 



152. Of Making Thin Sections of Dry Bone. For obtaining thin 

 sections of bone, a totally different process is requisite. In the first 

 place, a section as thin as possible is removed from the bone with 

 the aid of a thin sharp saw, pi. XXI, fig. 138. This may be made 

 somewhat thinner by a file, and afterwards ground down to the 

 required degree of tenuity upon a hone. The best stones for this 

 purpose are the Arkansas oil stones or the Turkey stones which have 

 been ground perfectly flat. The section may be kept in contact with 

 the stone by the pressure of the thumb or finger, or with a piece of 



