METHODS OF STAINING. 427 



fibres a dark blue, nearly black, color. If it has been preceded by 

 a stain with neutral carmine, the axis-cylinders of the nerve-fibres 

 will be stained red, and the nuclei of the nerve-cells will also 

 appear red. 



14. Golgi's Methods. These methods have yielded most excel- 

 lent results in the study of the central nervous system, the dis- 

 tribution of the peripheral nerves, and the delicate terminations 

 of the ducts of glands ; e. g., the bile-capillaries. The methods 

 must be regarded as special procedures in such studies, and can 

 but be referred to here. They all depend upon hardening in some 

 chromium salt, with or without the addition of osmic acid, and the 

 subsequent impregnation with silver nitrate. A precipitate is thus 

 produced on or within certain of the elements in the specimen, giving 

 them a dark-brown or black color. The methods are capricious, 

 and not all of the tissue-elements of like character in the specimen 

 are rendered prominent. This is an advantage, but necessitates 

 a degree of care in the interpretation of the results. Furthermore, 

 irrelevant precipitates may form in the tissues which have no 

 definite relations to any structure. Considerable practice is, there- 

 fore, required for the successful employment of all these methods, 

 not only for a satisfactory execution of the manipulations, but also 

 in the study of the results. The methods have no value for the 

 study of cell-structure, since the whole cell is either covered or 

 filled with the precipitates formed during the impregnation with 

 silver. 



Golgi has divided his methods into three groups : the slow, the 

 rapid, and the mixed. For the details of these methods and of 

 the various modifications introduced by different investigators the 

 student is referred to the journals on microscopy. It must suffice 

 to state here that the slow method begins with a hardening of the 

 tissues in a 2 per cent, solution of potassium bichromate, which is 

 gradually raised to 5 per cent. This hardening takes from fifteen 

 days to three months. In the rapid method the tissues are first 

 hardened in a mixture of 4 parts of a 2 per cent, solution of potas- 

 sium bichromate and 1 part of a 1 per cent, solution of osmic acid. 

 The tissues remain in this mixture for from two to six days, when 

 they are ready for impregnation. For either method the pieces 

 of tissue should not be thicker than 1.5 cm. 



