2 ATLAS OF NERVE CELLS 



For all these methods we are indebted to Professor Camillo Golgi, of Pavia. The rapid 

 method is the most important, and is the one which is now so extensively used. It is espe- 

 cially applicable to embryonic tissue, to nerve endings, etc. 



In addition to the above, the writer has devised two modifications which are quick and 

 yet avoid the use of osmic acid, which is an expensive reagent and difficult to manage ; viz. : 



(d) The lithium bichromate method, in which lithium bichromate is used instead of potas- 

 sium bichromate. The results resemble those of the long method, but the time of hardening 

 is reduced from nearly a month to two days. 



(e) The formalin-bichromate method, in which a mixture of potassium bichromate and 

 formalin ( = 40% solution of formaldehyde) is used in hardening. This also requires little 

 time to harden. It is more certain than (d}. Both (d) and (e) are especially applicable to 

 adult brains. 



The actual method of procedure in making the preparations for the atlas will now be 

 given. Certain of the details were rendered necessary by the nature of the material and by 

 the fact that the preparations were to be photographed. 



It will be most convenient to deal first with the preparations made from human embryos 

 and by means of the rapid method, as these constitute the bulk of the preparations used. 



The first essential is that the material be tolerably fresh. All the material which yielded 

 good results was obviously, from its general appearance, in good condition. Exact data are 

 not available, but in general human embryos of seven and eight months, if kept cold, probably 

 remain in fairly good condition for at least 24 hours after death. 



The hardening process. If the entire embryo is at hand, two courses are open : the 

 brain may first be injected in situ with hardening fluid, or may be immediately removed and 

 suitable pieces placed in the fluid. Injection was tried on one brain only, but the results war- 

 rant its further trial. In this case the fluid used was potassium bichromate 5% i vol. + osmic 

 acid 2 % i vol., the solution being made especially strong in osmic acid to allow for its sub- 

 sequent dilution in the tissue. About 100 cc. were injected into each carotid, and, when the 

 brain was opened an hour afterward, brown spots throughout the tissue showed where the 

 fixation had begun. The result was that in places groups of cells lying close to large 

 blood-vessels were impregnated, proving that the injection aided by infiltrating the tissue with 

 larger quantities of bichromate thus securing the formation of more silver chromate in these 

 places. Had the whole tissue been uniformly fixed in this way, which would have required 

 a much larger quantity of fluid, the result would probably have been still better. 



After the injection is completed and an hour or so allowed for fixation to take place, 

 the brain is removed and suitable pieces placed in the hardening fluid. These pieces should 



