PART II.
THE TREATMENT OF THE MATERIAL.
INTRODUCTION. The material obtained by autopsy, surgical operation, curettage, excision, spontaneous discharge, animal experimentation, etc., may be examined microscopically in the fresh state, or prepared for microscopic examination by methods of fixation, hardening and imbedding. The methods necessary for such histologic studies of pathologic material are given in the following pages, arranged as far as possible in their logical order. Only those methods are given that, in the light of the writer’s experience, yield the best results, from the standpoints of economy of time, labor, and expense, and perfection of result. The number of histologic methods contained in the literature is so great that it is out of the question for the student or practical worker to try out all of them. To give all of these methods would create confusion. I have attempted to avoid this by giving in full detail only those which in our laboratory experience have yielded the best results. So many methods represent but slight variations of some original method, and in the great majority of cases these variations add so little or nothing of value to the original method that in such cases the latter alone is given in full, with references only to the variations of the method. The individual equation plays such a large part in the judgment of laboratory methods that allowance has been made for this when certain variations or alterations in original methods have been strongly recommended by expert laboratory workers.
The purpose of the microscopic examination is the revealment of pathologic changes too small to be recognized by the naked eye, and the securing of a diagnosis that cannot be made macroscopically, as well as the confirmation of diagnoses based upon the gross appearances. Aside from these more immediate practical considerations, the microscopic examination of tissues is concerned with the solution of etiologic and pathologic problems, and the extension of our knowledge of disease. The aim of pathologic technique is the fixation of tissues for microscopic examination in such a manner that all of the morphologic and chemical elements and constituents of the tissue are perfectly preserved, so that with differential staining methods they are all brought out with sufficient contrast to be readily and correctly identified. In a certain number of methods this ideal is attained, and to Weigert, more than to any other worker in the field of pathologic technique, are we indebted for such ideal methods.
The choice of methods will depend upon the source and nature of the material, the object of the examination, the time-element and the degree of responsibility involved. The cellular elements of all pathologic fluids, secretions and excretions should be examined in the fresh state as well as in fixed preparations. For the demonstration of various chemical and morphologic features that are lost or altered by processes of fixation and imbedding, and when a rapid diagnosis is required, the examination of the material in its fresh state or by the freezing method is indicated. When the freezing method cannot be employed because of the changes in cells and tissue produced by it, when very thin or serial sections are desired, when a rapid diagnosis is not required, and when a very careful and minute study is desired, with the application of various staining methods, then the material should be fixed, hardened and imbedded and cut upon a microtome. Whatever method is chosen, it must be borne in mind, particularly in practical diagnostic work, that the portions chosen for microscopic examination must represent the characteristic anatomic structures of the tissue or organ, that living tissue be included, that the pathologic condition be represented both in its fully-developed state and at the transition-border between it and the healthy tissue, and that when sections are cut the block or tissue must be so oriented as to give the most comprehensive view of the tissue and the pathologic process. To accomplish this fully it is often necessary to make a number of blocks representing different areas of the material, and to cut these in different planes.