INTRODUCTION. xxxi 



that of man. The tissue to be hardened ought to be cut into small pieces half to one inch 

 square by means of a sharp razor. If foreign matters are to be removed from the tissue 

 e.g. the contents of the gut do not wash them with water, but with a stream of normal salt 

 solution (p. xxx). Place the tissues in a relatively large quantity of the hardening fluid, at 

 least twenty times the bulk of the tissue. Change the fluid at the end of the first, third, 

 seventh, and tenth days. It is better to place the tissues in a shallow but wide bottle. See 

 that the tissues do not stick to the wall of the vessel, else the hardening fluid is prevented 

 from having access to them. Cotton wool moistened with dilute alcohol answers admirably 

 as a bed for the tissues to rest on and to separate one piece from another. Place the 

 bottle in a cool place. The hardening will be effected in from one to four weeks. At the end 

 of this time remove the tissues ; wash them thoroughly in water, or let them steep in a large 

 volume of water for several hours to get rid of the hardening reagent, as in some cases e.g. 

 chromic acid its presence interferes with the process of staining. Place the tissues in equal 

 parts of methylated spirit and water for two days, and then transfer them to methylated 

 spirit to complete the hardening. If the spirit becomes cloudy it must be changed. This 

 keeps the preparations until they are required for sections. Instead of merely placing the tissue 

 in the hardening fluid, in many cases it is advisable to suspend it by means of a thin cord in 

 the fluid, which permits the fluid to penetrate the tissue more rapidly. 



The process of changing the various hardening fluids on certain days appears tedious, 

 but it is not so in reality, if the tissues to be hardened are placed on a shelf by themselves. 

 Chromic acid preparations require to have the fluid changed more frequently than when 

 Miiller's fluid is employed. For convenience, therefore, they may be placed by themselves. 

 On all occasions label each bottle, and note the tissue, from what animal it is taken, the name 

 and strength of the hardening fluid, and the date on which it was changed. Attention to 

 these apparently small details is the necessary condition for successful histological work. 



HARDENING SOLUTIONS. 

 The following are the solutions in most common use. 



A. THOSE WHICH HARDEN TISSUES WITHOUT COLOURING THEM. 



1. Chromic acid 1 per cent, solution. Dissolve 10 grms. chromic acid in 1,000 c.c. water. 

 So strong a solution is not used, as it renders the tissues too brittle. It is convenient to keep 

 a strong solution, which can be diluted as required. A \ or per cent, solution is generally 

 used. 



2. Chromic Acid and Spirit Solution. This is one of the most useful solutions. Make a 

 per cent, solution of chromic acid, either by diluting the above, or dissolving one gramme of 

 chromic acid in 600 c.c. of water. Take one part of this, and add two parts of methylated spirit. 

 It is better to prepare this mixture as it is required. As heat is developed by the mixing, the 

 tissues must not be put into the mixture till it becomes cool. It hardens in from seven to 

 ten days, according to the size and nature of the tissue. 



3. Potassic Bichromate Solution. Dissolve 20 grms. in 1,000 c.c. water. The solution is 

 most easily made with warm water, but it must be allowed to cool before it is used. If the 

 solution is changed every four days it hardens tissues in three to four weeks, but usually a 

 longer time is required, as it is not necessary to change the fluid so often. 



