146 Journal of Agricultural Research Voi. xvin. No. 3 



One of the difficulties confronting workers with nerve tissues is that in 

 a short time post-mortem changes set in, and the beginning dissolution 

 of nerve tissues may appear. Only the experienced technician accus- 

 tomed to handling nerve tissues can apprehend the consequences. It is 

 for this reason that the details of neurological technic will be described 

 to enable the reader to follow the various steps necessary in bringing out 

 all the details of the finer cytological changes which may occur in the 

 tissue as the result of disease. 



IMPORTANCE OF PROPER FIXATION 



Before proceeding to make a post-mortem examination, proper fixing 

 solutions must be ready to avoid all danger of disintegration. Fixation 

 is a process by which the tissue is quickly killed and its structure rendered 

 permanent. 



The fixing agent must have the power to penetrate quickly before 

 post-mortem changes have begun, as well as to give permanent results 

 without distorting the shape, size, and position of the tissue elements. 

 A fixing fluid of alkaline reaction should be avoided, since it tends to 

 dissolve certain structural constituents rather than to fix. Some 

 reagents, such as alcohol, while fixing rapidly cause a violent shrinkage 

 of the tissue by inducing an unbalanced exosmosis of the fluid cell con- 

 tents. In this way they bring about the shrinkage or collapse of the 

 cells, which is decidedly objectionable. 



The aim therefore should be to select fixatives in which the ingredients 

 are mixed in such proportions that the swelling tendency of one will 

 counteract the shrinkage tendency of the other. In removing nerve 

 tissue, care must be taken that the portions exposed do not become dry 

 and that the parts removed are not crushed or stretched. No single 

 fixing fluid and no single staining process suffices to bring out finer 

 structural changes. The neuroplasm of the ganglion cells is diffe: it 

 from the neuroplasm of the nerve fibers not only in its appeara ce, 

 composition, and behavior toward different fixatives but also in its 

 affinity for different stains. Chromatophil granules are present in the 

 ganglion cells but not in the ner\^e fibers. Myelin is present in the nerve 

 fibers, but not in the nerve cells. Held has pointed out that chro- 

 matophil granules are brought out by the treatment of nerve tissue with 

 alcohol or certain other fixing fluids. These appear according to their 

 treatment as fine or coarse objects but are not visible in fresh nerve cells. 

 The finer cytologic changes which are at the bottom of nerv^ous diseases 

 can be demonstrated only by modern methods of staining. 



The structural distinction of nerve cells and nerve fibers is made as a 

 matter of convenience for classification, description, and anatomical 

 correlation. In reality the nerve cell and the nerve fibers constitute col- 

 lectively the unit of the nervous system. This unit, called the neuron, is 



