POSITION OF KNIFE IN SECTION CUTTING 463 



holder. These (Xeumayer's) wedges, are horseshoe-shaped, so that 

 they may be slipped round the central screw. They are made in pairs, 

 one member of each pair having the opening of the horseshoe at 

 the thin. end. the other having it at the thick end. The wedge with 

 the opening at the thin end is slipped under the knife-holder 

 (tltin end towards the operator), and operates to tilt up the back of 

 the knife. 



The sister wedge is then placed over the slotted stem or handle 

 of the carrier, thick end towards the operator, in order that the 

 binding-screw may have a horizontal surface to bear on. The 

 wedges are sold in sets of three pairs, of different degrees of bevel. 



This simple device is quite suijicient so long as the utmost pre- 

 cision of section-cutting is not required. For more elaborate work 

 it is convenient to employ a special knife-holder, which provides a 

 means of elevating or depressing the back of the blade by rotating 

 the blade round its axis. Similar contrivances have been described 

 by Dr. Hesse (in the ' Zeitschrift fur wissenschaftliche Mikroskopie,' 

 xiv. 1, 1897, p. 13; see 'Journal of the Royal Microscopical Soc.' 

 1897, p. 441), and by Prof. Apathy (' Zeitschr.,' xiv. 2, p. 15, and 

 'Journal,' 1897, p. 582). This last is rather complicated to work 

 with, and consequently the Naples Zoological Station has worked 

 out a new device, made by Jung, which it is hoped will meet all 

 requirements. This is the ' Model L ' of his price-list, and is 

 figured in the 'Journal,' 1899, p. 546. That of Hesse is very 

 simple, and ought to be quite sufficient where no considerable 

 change of tilt is likely to be required. It is made by Jung. 



Before leaving this part of the subject it appears advisable to 

 consider briefly the question of knife-position in general a matter 

 011 which success or failure in section-cutting may often entirely 

 depend. 



The position of the knife should be varied according to circum- 

 stances, both according as to its slant or obliquity in relation to the 

 line of section, and as to its tilt, or the elevation of its back relatively 

 to its edge. 



As regards slant the slanting position, fig. 391, is adapted for 

 cutting soft and watery objects, not imbedded, and tissues imbedded 

 in celloidin, or the like ; for these cannot be cut with the knife 

 placed transversely. It is also frequently indicated for paraffin 

 objects ; but 011 this head no general rule can be laid down. The 

 transverse position, fig. 392, is indicated for cutting paraffin sections 

 by the ribbon method (see below, Imbedding Methods, Paraffin), and 

 also frequently for cutting loose sections by the paraffin method. 



As regards tilt : (1) The knife must always be tilted enough to 

 lift the under facet of the edge clear of the tissue as it passes over 

 it, for if not the tissues will be crushed by it as it passes over 

 them. (2) It must not be too much tilted, or it will not bite, but 

 will act as a scraper. Prof. Apathy, who has investigated the 

 subject in an instructive paper in the ' Sitzber. d. med.-naturw. 

 Section d. Siebenburgischen Museumvereins, Kolozsvar,' xix. 1897, 

 H. 7, concludes as follows: (1) The knife should always be tilted 

 somewhat more than enough to bring the under cutting-facet of the 



