14 HISTOLOGICAL TECHNIC 



specimens. It is then transferred to some solvent of paraffin. 

 Some of the solvents used are xylol, oil of cedarwood, chloroform, and 

 toluol. Of these the best are perhaps xylol and oil of cedarwood. 

 The tissue should remain in either of these for several hours, or until 

 the tissue becomes more or less transparent. It is then placed in 

 melted paraffin, in the paraffin oven, for from one to three hours, 

 according to the size and density of the specimen. This allows 

 the tissues to become impregnated with the melted paraffin. The 

 paraffin should be changed twice. 



In case of very delicate tissues it is w^ell to transfer them from 

 the absolute alcohol to a mixture of equal parts absolute alcohol and 

 xylol for a short time before putting them into the pure xylol. In the 

 same way a mixture of equal parts xylol and paraffin may be used 

 before putting the tissues into pure paraffin. 



For hardening the paraffin in and around the tissue a very con- 

 venient apparatus consists of a plate of glass and several L-shaped 

 pieces of iron or lead. Two of these are placed on the glass plate in 

 such a manner as to enclose a space of the desired size. Into this are 

 placed the specimen and sufficient melted paraffin to cover it. Both 

 glass and irons should be smeared with glycerin to prevent the par- 

 affin from adhering, and should be as cold as possible, so that the par- 

 affin may harden quickly. The same paper boxes described under 

 celloidin embedding ma}^ also be used for paraffin. Another good 

 method for small pieces of tissue is to place the specimen in paraffin 

 in an ordinary watch-glass which has been coated with glycerin. 

 Both paper-box and watch-glass specimens are immersed in cold 

 water as soon as the surface of the paraffin has become hard. After 

 the paraffin has hardened any excess may be cut away with a 

 knife. 



Paraffin-embedded specimens may be kept indefinitely in air. 

 For section cutting, the block of paraffin is attached to a block of 

 wood or of vulcanite or to the metallic block-holder of the microtome. 

 This is done by heating the block-holder, pressing the paraffin block 

 firmly upon it, and then dipping the whole into cold water. 



VI. Section Cutting 



The older method of making free-hand sections with a razor has 

 been almost completely superseded by the use of a cutting instru- 

 ment known ns the microtome. This consists essentially of a clamp 



