CHOICE OF METHODS XIX 



of Cartilaginous Skeletons, Ossification centers, Blood Vessels and so on almost 

 without end. 



4. To Employ the More Routine Method of Fixation and Staining 



Here there is wide latitude of choice. For some purposes thin Smears are 

 just fixed and stained without resort to sectioning. In the case of the denser 

 tissues which must be cut in sections one first has to decide which of many 

 Fixatives is likely to give the best results. Then, whether fixation is to be by 

 immersion or injection has to be determined. 



The purpose of fixation by vascular injection is to bring the fixative into close 

 contact with the tissues as they exist in the freshly killed animal without sub- 

 jecting them to mechanical trauma or disturbing their topographic relations one 

 to another. In choosing this procedure it is well to remember: (1) That it is 

 usually necessary first to wash out most of the blood by perfusion with physio- 

 logical salt solution for otherwise the fixative often clogs the vessels. This wash- 

 ing unfortunately also facilitates chemical change. (2) That, even when it is 

 not done, the concentration of the fixative about the cells is gradually increased 

 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 with; 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 Caraoy'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 which 

 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. 



