CHOICE OF METHODS 5 



and at different rates, rapidly in highly vascularized tissues (kidney, liver, etc.) 

 and very slowly in avascular ones (epidermis, cornea and cartilage). The time 

 for chemical change before fixation is therefore variable depending upon the 

 tissue. (3) That the pressure may bring about an unnatural swelling of the 

 tissues so located that they can enlarge, especially the abdominal organs as 

 compared with brain and bone marrow which are confined within rigid walls. 



Fixation by immersion is the usual and easiest method. If small pieces or 

 thin slices are used the preservation is quicker and more uniform than by vascu- 

 lar injection. The cells are suddenly killed while active. The factor of slow 

 death at uneven rates, present in supravital examinations, does not have to be 

 reckoned wdth; but many precautions are required. Under Fixation is given a 

 general account of the procedure. Under the several organs, Lungs, Small 

 Intestine, Skin, etc., some special suggestions are provided. There are many 

 fixatives to choose from. For routine purposes Zenker's Fluid as originally 

 described or in one of its numerous modifications is suggested. Bouin's is also 

 a very popular fixative especially among dermatologists. Formalin is an ex- 

 cellent one. It is good practice to set aside some tissue in formalin for examina- 

 tion as may be needed later. Both formalin and alcohol are the most useful 

 fixatives preliminary to microchemical determinations. When preparations 

 must be made very quickly. Alcohol Formalin and Carnoy's Fluid are suggested 

 (see also Frozen Sections). For microincineration, formalin-alcohol is ordinarily 

 employed; but the Altmann-Gersh method of freezing and drying, by wliich 

 contact with fixatives is altogether dispensed with, is much less open to criticism. 

 Osmic acid containing fixatives penetrate poorly and are therefore only useful 

 for very small pieces of tissue. Regaud's fluid with subsequent mordanting in 

 bichromate is the best for mitochondria. Heat fixation is useful for blood cells. 

 Fixation in various vapors is called for in special cases. See Fixatives. 



After fixation some Washing of the tissue in water is necessary unless it has 

 been fixed in alcohol, Carnoy or similar mixtures. The next step is Dehydration 

 and a choice must be made between slow and rapid methods. Sometimes a 

 substitute for alcohol is indicated. If Imbedding is to be in celloidin Clearing 

 in a xylol-likc fluid is omitted and heating is unnecessary. There are many ways 

 of clearing preliminary to paraffin imbedding. In Sectioning the thickness 

 depends upon the purpose in view. Thick sections may be as necessary as thin 

 ones and serial sections are often required. In the Mounting of sections on 

 slides the use of water must occasionally be avoided. Numerous techniques are 

 applicable to the sections and are given individually later either under the head- 

 ing of the substance or structure to be demonstrated or under the name of the 

 technique or its introducer. For choice see Staining. 



Many beautifully stained sections of well fixed tissue arc of but little value, 

 because the investigator failed to note the exact location in the organ or tissue 

 whence they were excised and omitted to have the sections cut in the most 

 favorable plane. 



