



HISTOLOGICAL METHODS. 161 



are still better. They are, however, only suitable for making 

 sections of tissue held in the hand, their blades being too 

 thin for use with a microtome. For ordinary purposes it 

 is not necessary to have the razor ground flat on one side 

 (the right when the face is down). It does well enough 

 to have the blade concave on both sides, provided it be 

 broad enough to receive the whole section ere it come 

 against the thick back of the blade. If one attempt, how- 

 ever, to make large sections without the aid of a machine, 

 then a long broad-bladed knife, flat on both sides is neces- 

 sary, but it is only a waste of time to attempt anything of 

 the sort without a microtome for the purpose of keeping the 

 tissue fixed and the blade gliding steadily and evenly during 

 the process of section (see 302). 



Beer's knife is a useful instrument for puncturing the 

 cornea when a drop of aqueous humour is wanted. An 

 ordinary scalpel is not suitable for this purpose. Beer's 

 knife has a thin triangular blade tapering to a fine point. It 

 may also be used for making sections in place of a scalpel. 



FIG. 57. Beer's knife. 



295. Wetting the Knife The knife must always be 

 wet, in order that the slice of tissue may float over its surface. 

 a Water, b salt solution, c methylated or rectified spirit, d 

 absolute alcohol, are used for this purpose. Of these, c and 

 d are the best, but they are only to be used in cases where 

 spirit may be added to the tissue without altering it e.g., 

 when the tissue has been hardened in spirit, Mutter's fluid, 

 or chromic acid. 



296. Hand-sections. If the tissue and the knife be 

 both simply supported by the hands, the sections are, for 

 convenience, often termed hand-sections. The tissue is 

 held between the thumb and fore and middle fingers of the 

 left hand, and the forefinger is used as a table for support- 

 ing the back of the knife. The knife should never be held 

 like a ptn, and it should never be pushed but always drawn 

 from heel to point obliquely through the tissue. . The sec-: 



If 



