The Impregnation of Neurofibrils *

The importance of neurofibrillar impregnations in the study of the nervous system needs no emphasis. However, the silver methods currently used involve such a complex of variables that the results are by no means constant. The present communication is devoted to the problem of interpretation of results, together with certain fundamental relations between brain tissue and silver solution. The most important practical general method for the study of neurofibrils in pathological material is the use of ammoniacal silver solution on frozen sections of formalin-fixed material. Ammoniacal silver was first introduced by Fajerstajn,4 although Cajal7 daims some priority. However, the method of Bielschowskyt using silver hydroxide has remained, with some variations, the standard in current use. The employment of sodium carbonate instead of sodium hydroxide, introduced by Hortega, was a step of considerable importance. The resulting solution of silver carbonate is extremely delicate and capable of a very wide variety of uses. Hortega'se own summaries of his methods are somewhat difficult of access, but the technical details have been made available in English by his pupils."' 8 The paper of Kubie and Davidson' offers the most complete available theoretical discussion of the chemistry of ammoniacal silver solutions, their methods of preparation, and of their properties. These authors bring out that the active principle in all such solutions is the complex ion [Ag(NH3)2]', but the activity of this ion is largely determined by the other constituents of the solution. They point out that the silver carbonate and silver hydroxide are essentially interchangeable, provided that due allowance is made in steps of technic for the much greater vigor of the hydroxide. The manual of Cajal and Castro7 summarizes many such methods wherein silver hydroxide can be substituted for silver carbonate. In the present studies silver solutions made with silver carbonate