166 PRACTICAL HISTOLOGY. 



the machine will be more readily comprehended after a 

 description of the first. 



303. When the microtome is used without freezing, the 

 tissue may be imbedded in paraffin or in carrot. 



a. When paraffin is employed the temperature at which 

 the paraffin is melted should be just sufficient for the pur- 

 pose and no more, in order that the tissue may be heated 

 as slightly as possible, and also in order that the paraffin 

 may, on cooling, shrink to the smallest possible extent. 

 With a pair of forceps dip the tissue previously exposed 

 to the air for a short time in order to dry its surface into 

 the melted paraffin, at once withdraw it, and, when the 

 paraffin crust has cooled, replace it permanently in the well 

 of paraffin, close to the margin from which the face of the 

 knife is to moved in the process of section. The side 

 nearest the operator is most convenient. A mixture of 

 equal parts of bees'-wax and sweet oil is much recommended 

 for the same purpose as the paraffin in the above case. 

 Certainly the wax and oil mixture is excellent for imbedding 

 tissues, and it can be cut with ease. But it melts at a 

 higher temperature than paraffin, and, owing to the great 

 thermal expansion, it retracts from the side of the tube of 

 the machine, and so the wax cylinder becomes loose. The 

 only way in which this can be prevented is by heating the 

 machine to a like temperature before introducing the wax. 

 This is tedious ; and, inasmuch as it is unnecessary in the 

 case of paraffin, this is to be preferred. Even with this, 

 however, the paraffin cylinder is apt to become a little loose, 

 and to turn round in the machine : hence it is important 

 that there be an eccentric hole in the brass plug (K, Fig. 59) 

 with a projecting wooden pin fixed in it. The rotation of 

 the paraffin cylinder is thus prevented. 



b. When carrot is used a cylinder of it to fit the well of 

 the microtome must cut with a cork-cutter, and the tissue 

 imbedded as already described in 298. 



304. The knife must have a back with straight edges, so 

 that it may not tilt in the process of section. The surface 

 that rests on the plate should always be hollow, and it is 

 convenient to have the upper surface the same (R, Fig. 59), 



