4 NERVOUS SYSTEM OF VERTEBRATES. 



make up a composite picture approaching completeness. The 

 method when used alone always has the disadvantage that the 

 observer can never know when he has obtained an impregnation 

 of all the elements present in the organ which he wishes to study. 

 The peculiar value of the method Hes in the fact that the nerve 

 elements themselves are brought to view. Not only can the form 

 of the cells and their dendrites be seen but the neurites can be 

 traced directly from their origins to their endings. The polarity 

 of form of the nerve .elements is shown and the direction of flow of 

 impulses can be inferred with little danger of error. The method 

 is especially adapted to the study of the central nervous system 

 and gives facilities for determining the functional relations of 

 centers and tracts scarcely afforded by any other anatomical 

 method. It is equally trustworthy for the peripheral nervous 

 system wherever clear pictures can be secured, but the technical 

 difficulties are greater on account of the conditions of sectioning 

 and the formation of precipitates between the tissues. 



The method of staining myelin sheaths commonly known as 

 the method of Weigert, who introduced it in 1881, differs widely 

 from the method of Golgi in that it stains non-nervous elements 

 only. The picture gi\en by this procedure is complete so far 

 as the myeUn sheaths are concerned. Since the gray matter 

 and the non-myelinated tracts are unstained, the myelinated 

 tracts can be traced with relative ease. The method is especially 

 adapted to the central nervous system where it gives quickly and 

 easily the course and general topographical relations of the fiber 

 tracts. The method is limited in its usefulness because it does 

 not give the origin or ending of neurites or the course of non- 

 myelinated tracts, fibers or collaterals. Since neurites may run a 

 longer or shorter distance before receiving their myelin sheaths and 

 since the terminal branches of neurites run an unknown distance 

 after passing the end of their sheaths, the study of the sheaths may 

 lead to erroneous conclusions as to the groups of cells from which 

 given fibers arise or in which they end. Since in recent years 

 it has become possible to apply the method to material fixed in 

 formalin or in Flemming's fluid, even better preparations are 

 obtained than formerly and with greater ease. When the usual 



