18 MANUAL OF HISTOLOGY. 



tion may be made of the whole organ at one sweep of the knife. 

 The knife and frame are modifications of those devised by Dr. E. 

 Curtis of this city, and the section-cutter and box are not dif- 

 ferent in any essential particulars from those in common use. 



They are larger, however, and the indicator, Gr, enables the 

 observer to determine with accuracy the thickness of his sec- 

 tions. Thus, in my own instrument thirty-one turns of the 

 milled head drives the plug forward one inch. 



Each revolution consequently drives the specimen forward 

 ^ T inch. Now, the circumference of the milled head is marked 

 off into thirty divisions. 



When the indicator marks that the plug has been driven 

 forward one division, the distance traversed will be -fa inch. 



It is easy, therefore, to determine the thickness of any sec- 

 tion with considerable accuracy. 



When it is desirable to put the instrument in use, the 

 plug that is to be used is well oiled, as also the thread of the 

 driver, and the metallic box is filled with a mixture of ice and 

 snow. 



It is necessary to be particular and oil the bearings 

 thoroughly, else they will bind and the instrument will be 

 clogged while the freezing process is going on. The usual 

 plan is to soak the tissue (as Dr. Pritchard suggests) in a 

 thick solution of gum, which cuts like cheese when frozen. 

 The soaking should continue for a number of hours, say until 

 the next day. 



When the tissue is ready, a thick solution of the guru 

 should be poured into the well and the tissue held until it is 

 fixed by the ice. Some non-conductor is to be placed over 

 the well as soon as fixation has commenced, in order that ac- 

 cess of heat may be prevented. 



If ice is used it should be ground up finely and then packed 

 tightly about the well ; snow is better. The whole process 

 takes only ten or fifteen minutes. The freezing section-cutter 

 is of use when we are desirous of making a rapid examination 

 of fresh tissues. 



It is obvious that they are seen under more natural circum- 

 stances than when they have passed through the bichromate 

 or chromic acid solutions, or alcohol, all of which cause more 

 or less change in such delicate substances. 



It has been hoped that by the freezing method we should 



