336 CENTRAL NERVOUS SYSTEM. 



before applying balsam, or other mediu.ni, and a cover. For 

 celloidin sections you may use one of the methods described 

 in 325-328, or for large sections the caoutchouc or gutta- 

 percha method of Threlfall or Frenzel, or the gelatin method 

 of Fol, or Minot's shellac method. This is as follows : You 

 arrange the sections on the slide, and cover them with a layer 

 of 10 to 12 per cent, solution of shellac, then warm to 30 or 

 40 C., until the shellac appears dry, clear with clove oil, and 

 mount in balsam (Zeit. f. iviss. Mik., 1886, p. 175). This pro- 

 cess is applicable either to paraffin or collodion sections. 



676. Serial Section Mounting. Besides the ordinary methods for 

 mounting small sections, there are certain processes specially adapted to the 

 mounting of very voluminous sections. One of these is GIACOMINI'S collodion- 

 gelatin method, which is certainly a valuable one. But it is hardly more 

 than a macroscopic method, principally adapted for the study of the coarser 

 topography of the brain, and as such may be passed over here, though I 

 would willingly include it did our shortening space allow. Descriptions of 

 the process are to be found in Gazzetta delle Cliniche, Nov. 1885 ; Zeit. f. 

 wiss. Mik., 1885, p. 531 ; and Traite des Meth. Techn., LEE et HENNEGUY, 

 p. 392. 



A further reason for passing over this process is that the same end may 

 be attained, in many cases more effectually, by the beautiful methods of 

 WEIGEBT ( 327), STEASSEE ( 316), and APATHY ( 326, 326a), which see. 



A useful method for small sections is that of OSBOEN, who arranges his 

 sections, wet with alcohol, in order on the slide, covers them with a cigarette 

 paper, and treats them with the clearing agent through the paper. 



For DEECZE'S method, see above, 672. 



Other Methods for Nervous Centres. 



677. The Half-Clearing Method. Since the days of Lockhart Clarke, 

 to whom the method is due, most observers have been struck by the wealth 

 of minute structure revealed by half-cleared sections. Detail that is invisi- 

 ble in perfectly cleared sections stands out in sculptural relief during the 

 period in which the tissues are only about half cleared. It is of course most 

 important to be able to preserve preparations permanently in this state. 



MBBKEL (Arch.f. mik. Anat., 1877, p. 622) gives the following instruc- 

 tions : Sections are dehydrated with alcohol of about 94 percent, (in which 

 they must remain for at least ten minutes), and then cleared with xylol, in 

 which they are examined. 



Certain elements of the tissues retain more obstinately than others the 

 small quantity of water that they bring with them out of the !>1 per cent. 

 alcohol. Now, as xylol is absolutely immiscible with water, it can exercise 

 DO clearing action on these, and they stand out holdly ill the picture by virtue 

 of the difference between the index of refraction of their contained water 

 and that of the xylol. The axis-cylinders are at til-stall that is visible ; after a 



