TREATMENT OF SECTIONS 55 



which 1 to 2 per cent, of acetic acid may be added at the 

 time of using, or 10 per cent, formalin, are suitable for 

 the purpose. The pieces of fresh tissue are placed in 

 either solution for twelve to twenty-four hours, washed in 

 running water, and then placed in absolute alcohol, or 

 passed through increasing strengths of alcohol as detailed 

 above. 



After hardening, the tissue may be preserved in 70 per 

 cent, alcohol. Miiller's fluid (potassium bichromate) and 

 chromic acid are not suitable for hardening tissues for 

 bacteriological work. 



After hardening, the tissue is embedded, in order to 

 prepare it for the section- cutting machine. 



The Freezing Method. The tissue can be frozen without 

 preparation beforehand, but where it has been hardened 

 it must be first freed from alcohol. The pieces of tissue 

 are placed in a wide-mouthed bottle, a funnel is stuck in 

 the mouth of the latter, and a stream of water from a 

 tap allowed to run in for one to two hours. The tissues 

 are then soaked in a mucilage of cane-sugar and gum 

 acacia (to which a little carbolic acid or a piece of thymol 

 should be added to prevent the growth of bacteria and 

 moulds) for twenty-four hours, and are then ready for 

 cutting. A piece of tissue is placed on the plate of the 

 freezing-microtome, a little mucilage added, so that it is 

 surrounded with this, and the whole frozen, the freezing 

 being carried out by spraying the under-surface of the 

 plate with ether. This is done by filling the bottle of the 

 apparatus with ether and working the bellows. The 

 sections are cut by working the knife, which should be 

 moistened with water, and the sections as cut are re- 

 moved with a moistened camel' s-hair brush, and immersed 

 in a dish of tepid water for an hour to remove the gum. 

 From time to time the bellows are worked to keep the 

 mass frozen. Compressed carbon dioxide is now often 

 used instead of ether; the jet of escaping gas impinging 

 on the under surface of the plate quickly freezes the 

 section. After the washing with water the sections may 

 be stained, or may be preserved in 70 per cent, alcohol 

 for future examination. 



The freezing process is indicated when rapid production 

 is essential. The sections stain readily. It is frequently 

 used during the actual progress of operations. It has 



