CHAPTER VIII. -85 



that contain much chitin or much muscular tissue ; and better with 

 a slowly working sliding microtome than with a quick-working 

 Rocker or the like. Soft masses such as gelatin or celloidin cut wet, 

 can only be cut with the slanting knife. The slanting position 

 causes less compression of sections than the transverse one. It has 

 the defect of producing rolling in paraffin sections more easily than 

 the transverse position. The latter is the proper position for 

 cutting ribbons of sections from paraffin. 



By the tilt of the knife is meant the angle that a plane passing 

 through its back and edge makes with the plane of section : or, 

 practically, the greater or less degree of elevation of the back above 

 the edge (it is not to be confounded with the inclination of the long 

 axis of the knife to the horizon ; any accidental inclination that this 

 may have is a matter of no moment). 



The question of the proper tilt to be given to the knife under 

 different circumstances has been investigated by APATHY, loc. cit. 

 supra. He concludes (1) The knife should always be tilted some- 

 what more than enough to bring the back of the under cutting-facet 

 clear of the object. (2) It should in general be less tilted for hard 

 and brittle objects than for soft ones ; therefore, cceteris paribus, 

 less for paraffin than for celloidin. (3) The extent of useful tilt 

 varies between and 16 or occasionally 20. (4) Excessive tilt 

 causes rifts (longitudinal) in the paraffin, also furrows that in bad 

 cases split up the section into narrow ribbons. It also makes 

 sections roll. Also it may cause the knife not to bite, thus causing 

 sections to be missed. Or it may give an undulatory surface to the 

 sections, owing to vibrations set up in the knife, which may be 

 heard as a deep humming tone. Further, I would add, excessive 

 tilt may cause the knife to act as a scraper, carrying away portions 

 of tissue bodily from their places. Excessive tilt may often be 

 recognised by the knife giving out a short metallic sound just as it 

 leaves the object. For knives with plane under-surfaces it is seldom 

 advisable to give less than 10 tilt. Knives with concave under- 

 surfaces, on the contrary, may require to be placed almost horizontal. 

 Jung's knife-holders give mostly a tilt of about 9, which is only 

 enough for cutting ribbons with hard paraffin. 



A knife with too little tilt will often cut a second section, or frag- 

 ments of one, without the object being raised, showing that during 

 the first cut the object was pressed down by the knife, and recovered 

 itself afterwards. This fault is denoted by the ringing tone given 

 out by the knife on passing back over the object before the latter 

 is raised. Such a knife gives out a dull rattling sound whilst 



