408 CENTRAL NERVOUS SYSTEM. 



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. wiss. Mik., 1886, p. 175). 

 This process is applicable either to paraffin or collodion sec- 

 tions. 



778. 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 HENNEQUY, 

 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 

 WEIGEET ( 340), STKASSEB ( 326), and APATHY ( 338, 339), which see. 



A useful method for small sections is that of OSBORN, 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 DEECKE'S method see above, 743. 



Other Methods for Nervous Centres. 



779. 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. 



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

 tions : Sections are dehydrated with alcohol of about 94 per cent, (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 94 per cent, 

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

 no clearing action on these, and they stand out boldly in 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 first all that is visible ; after 

 a time the ganglion-cells appear with their processes. Nuclei, vessels, and 

 whatever else may be in the preparation are totally invisible. Merkel thinks 

 this method superior to all others for the study of the distribution of nerve- 

 fibres. The preparations are not permanent, though they may be kept for 

 some weeks by mounting them in Canada balsam. When a preparation 

 (either in xylol or balsam) has become so transparent as to be of no further 



