THE STUDY OF THE NERVOUS SYSTEM. 3 



topographical relations of nerve centers and fiber tracts, and for 

 the purpose of reconstructions by which these relations can be 

 shown on an enlarged scale in wax models and the like. Series 

 of sections prepared with such a stain as Delafield's hematoxylin, 

 well differentiated, remain one of the best means of studying 

 general anatomical relations, especially in the central nervous 

 system. 



Scarcely had this general method been developed when special 

 methods were devised to gain more exact pictures of nerve struc- 

 tures, especially by taking advantage of the selective activity of 

 the chemicals to be employed. First among such methods was 

 the impregnation of nerve elements by metallic salts. Results 

 obtained by this method were published by Golgi in 1875, but the 

 method did not come into general use for a number of years. The 

 reasons for this were that the procedure followed by Golgi was 

 slow, requiring several months for its completion, and the impreg- 

 nation was more or less uncertain, while the method of Weigert 

 which was soon introduced promised results more quickly and 

 with less labor. About 1890 new procedures which were both 

 more rapid and more certain in their action were brought forward 

 by Cajal, Cox and others, and since that time the method of 

 metallic impregnation has been more and more widely used. 

 This technique has given some of the most valuable contributions 

 to our knowledge of the nervous system and is still among the 

 most important methods for research. The advantage of the 

 method lies in the fact that it gives incomplete pictures. Few 

 elements are stained, often in great detail; and these are not 

 obscured by the richness and confusion seen in complete pictures. 

 The coloring of the tissue is not a true stain but an impregnation 

 with a metallic salt. Although neuroglia, capillaries and other 

 elements may at times be impregnated, the procedure peculiarly 

 affects the nerve elements. Of these only a few are impregnated, 

 the selection apparently being due in some way to the physiological 

 state of certain elements. The elements which are thus isolated 

 may be studied with great certainty and completeness, but the 

 full study of all the elements in a given region requires a suffi- 

 ciently large number of preparations to enable the observer to 



