GENERAL METHODS OF INVESTIGATION, BY S. STRICKER. xxix 



should be applied in solution, containing 0'25 to 2 per cent., 

 and the perfectly fresh tissue ought to be placed in a large 

 volume of the acid solution. The skin and all mucous mem- 

 branes, the intestines, bladder, and conjunctiva, become in the 

 course of a few days sufficiently hard to permit sections to be 

 made ; and even this period can be shortened by removing the 

 preparation from the chromic acid solution, and immersing it 

 in alcohol, where it may remain for twenty-four hours. The 

 proper hardening of the brain and spinal cord, however, re- 

 quires a longer time. Large portions generally putrefy in the 

 centre, though they harden at the surface. These parts of the 

 nervous system should therefore be cut into small fragments. 

 Here also the subsequent application of alcohol proves of great 

 service. The bichromate of potash acts in the same way as 

 chromic acid, but much more slowly, the effect produced in a 

 few days by the latter requiring weeks with the former. At 

 the same time, the bichromate of potash possesses the very 

 great advantage that the tissues saturated with it do not be- 

 come friable. Recently, perosmic acid and chloride of palla- 

 dium have been recommended as means of hardening, the 

 solution containing from one-fifth to one-tenth per cent. 



Various forms of apparatus have been constructed, by means 

 of which fine sections can be made. It would be undoubtedly 

 a great step in advance, if they could be made in any way 

 which would render us independent of manual dexterity. But 

 up to the present time these mechanical means have not 

 attained sufficient excellence to lead to their general adoption. 

 Sections are therefore still always made by the hand, and their 

 beauty depends on the greater or less skill of the operator. 

 The knives employed should always be of the best quality, and 

 extremely sharp ; scalpels will be found to be best adapted for 

 objects that have been hardened by boiling, whilst large flat 

 blades are more appropriate for those that have been hardened 

 in fluids. The sections, when made, may either be examined 

 without further addition ; or they may be first prepared by 

 means of needles, or be freed from adhering or imbedded 

 morphological elements by the frequent use of a soft brush, 

 or by blows with a delicate rod, or by shaking them in small 

 test tubes. If the tissues are friable, or too small to be seized 



