84 



223. Procedure. Connect a canula with the artery supply- 

 ing the alimentary canal (superior mesenteric) or the brain (caro- 

 tid) and inject distilled water until the water flows out of the re- 

 turning vein colorless. Then immediately inject the silver solution 

 until it runs from the vein. After a minute or two follow w r ith 

 distilled water or physiological salt solution. Place the intestines 

 and mesentery in water and expose them to the light until they become 

 slightly browned. Strips of the muscular coat of the intestines, 

 especially of the rabbit, will show capillaries well. Veins and arteries 

 side by side may be found in the mesentery. If the brain vessels are 

 injected one can get admirable preparations showing nuclei as well as 

 cell outline by staining in hematoxylin. Mount in glycerin, or, if 

 desired, dehydrate and mount in balsam. The tissue may be kept in 

 50% alcohol or in 50% glycerin for several months before mounting 

 if it is kept in the dark. 



For large vessels and endocardial epithelium open the vessels 

 or the heart and silver as directed above for mesentery. It may be 

 necessary to make thin free-hand sections so that the preparation 

 will be thin enough for high powers. 



HISTO-CHEMICAL METHODS. 



There are special chemical substances which it is often desirable 

 to preserve and differentially stain. In most cases, the staining re- 

 actions are not specific enough to come under the category of micro- 

 chemical tests, the evidence gained being circumstantial or indirect, 

 the application of two or three different methods being sometimes 

 necessary for confirmation. Such methods may be spoken of as 

 histo-chemical rather than micro-chemical. 



A. Fats. (Lipoids). 



224. Free fats and lipoids are soluble in ether, chloroform, 

 absolute alcohol, xylene, benzene, and essential oils. As these are 

 necessary for paraffin and celloidin imbedding methods, especially 

 the former, the satisfactory preservation of these substances pre- 

 sents some difficulties. The use of the freezing microtome is there- 

 fore particularly called for. See, however, 227, 228. 



The stains applicable to the demonstration of fats are (a) stains 

 soluble in the fat solvents, e. g., Sudan III, and scarlet red; and (b) 

 such as depend upon the reduction of salts by the fats (e. g., osmic 

 acid and potassium dichromate) [29]. 



