Preservative media.—­Canada balsam may be prepared as follows:  Place some pure Canada balsam in a saucer, and cover with paper to exclude dust; dry it in an oven at a temperature of 150 deg.; when it cools, it will become hard and crystalline.  Dissolve this in benzole, and use in the same way as glycerine.

Dammar is now used as a substitute for Canada balsam.  By its use the tissues are rendered more transparent.  To prepare it, dissolve one-half ounce of Dammar rosin and one-half ounce of gum mastic in three ounces of benzole, and filter.  This may be used to mount unsoftened bone and tooth, hair, brain, and spinal column, and most tissues that have been hardened in alcohol or chromic acid, which require to have their transparency increased.

Glycerine is not adapted for white fibrous tissue or blood vessels, unless they have been hardened in chromic acid, as it causes the white fibers to swell up and lose their normal features.  Sections of liver, lung, skin, and alimentary canal show better in glycerine unless they have been stained.

Farrant’s solution may be substituted for glycerine in many instances, because of its feebler tendency to render the tissues transparent.  It consists of equal parts of gum arabic, glycerine, and a saturated solution of arsenious acid.  In mounting preparations with this medium, the covered object should be allowed to lie a day before the varnish is applied, so that the cover may be fixed, and thereby prevented from being displaced.  Rectified spirits may be used for mounting softened bone and tooth, and naphtha and creosote are useful for preserving urinary casts.

When the section is mounted in Canada or Dammar balsam, no cement is required, but for all other preservative media the margin of the cover must be covered with cement.  To do this, dry the edges of the cover thoroughly with bibulous paper, and paint a layer of gold size, allowing it to overlap the cover an eighth or sixteenth of an inch, then cover this with white zinc cement.

Preparation for mounting the different tissues.—­To obtain a section of bone or tooth requires a grinding down of the tissue until it is so thin as to be transparent.  A section should first be cut as thin as possible by a fine saw.  It should be attached by the flattest side to a piece of glass, and then ground down by a grindstone or by very fine emery, on a perfectly flat piece of lead.  When sufficiently thin and transparent, mount in rectified spirits or Dammar.  Sections of the tongue may be made by embedding in paraffin, and mounted in Farrant’s solution or glycerine.

Sections of the stomach may also be made by embedding in paraffin, but better ones can be made by freezing.  Farrant’s solution makes a good mounting.

The intestines also give a better section from freezing than by embedding, as the paraffin injures the villi; mount in the same medium as the stomach.