300 NORMAL HISTOLOGY. 



the razor, or microtome-knife, the stroke should be from point to 

 heel during both the forward and return motions. In cutting, 

 the edge should be used from heel to point, and this same motion 

 should be used in honing. A wire arrangement is usually furnished 

 with microtome-knives, which is intended for use while honing or 

 stropping. It serves to raise the back of the knife when the flat 

 side is sharpened, and should always be employed. Care must be 

 taken not to press the knife against the strop, as this is liable to 

 turn or blunt the edge. A few light strokes on the strop immedi- 

 ately after each day's use will keep the knife sharp and coat it with 

 a little grease, protecting it from rust. A microtome-knife should 

 never be allowed to rest with its edge on any hard surface ; the mere 

 weight of the knife is sufficient to spoil its edge. 



In cutting free-hand sections of fresh tissues the upper surface 

 of the razor should be kept flooded with normal (0.75 per cent.) 

 salt solution. The sections float in this fluid and are kept from 

 tearing. Each section should be removed by a single stroke of the 

 razor. When hardened specimens are cut, 80 per cent, alcohol 

 should be used instead of salt solution. 



Free-hand sections cannot be made either so thin or uniform as- 

 sertions prepared with a microtome, and these instruments are now 

 so cheap that they are universally used. There are three principal 

 forms: 1, freezing-microtomes ; 2, paraffin-microtomes; 3, micro- 

 tomes for cutting sections of tissues embedded in collodion. The 

 last are often fitted with attachments intended for use in cutting 

 frozen sections, and can also be used for paraffin. But the best 

 results are obtained by using instruments especially designed for 

 each purpose. 



1. Frozen Sections. — Freezing is usually employed when sections of 

 fresh tissues are to be made, but hardened tissues may be cut with 

 a freezing-microtome if the alcohol be first removed by soaking for 

 a considerable time in water. The tissue may be placed upon the 

 plate of the microtome in a little water or neutral salt solution ; but 

 a better method is first to soak the tissue in a syrupy solution of 

 gum-arabic, and to moisten the plate with the same before freezing. 

 This solution freezes in less coarsely crystalline form than water or 

 salt solution. 



When the tissues are frozen, thin sections are removed Avith a 

 quick forward and slightly oblique stroke of the knife. The motion 

 is intermediate between that of a plane and a single stroke of a saw. 



