116 MANUAL OF HISTOLOGY. 



it is to be immersed in water for a few hours ; then in 90 per 

 cent, alcohol, and then in a weak solution of gum-arabic, 

 which fills the interstices between the bundles, and finally in 

 strong alcohol (95 per cent.), which hardens the gum sufficiently. 

 The sections, cut as thin as possible, should be placed on a 

 slide to remove the excess of alcohol, which may be done with 

 filter-paper. A drop of water is then to be added ; about 

 the cover put a few drops of carbolized water ; remove to a 

 damp place. At the end of twenty-four hours the gum will 

 have dissolved, and then the glycerine may be allowed to enter 

 slowly without displacing the elements (Ranvier). 



In examining such cross-sections, the medullated nerves will 

 present various diameters, and the contour of the myelinic 

 sheath will vary in width and outline according as the cut 

 comes through the broadest part of the arrow-marking, or 

 through the thin overlapping parts. (See Fig. 43.) If the cut 

 chances to pass close to the annular constriction, no myeline 

 will of course be seen. For these reasons, the cross-sections of 

 such nerves, when stained with osmic acid, are very different. 



Modern conceptions of myelinic nerves. The specimens 

 that have been studied according to the methods given will not 

 have shown any termination of the nerves, or any division, 

 either into trunks of any considerable size or into the fibrils of 

 which they are said to be composed. They do, however, as we 

 have already said, dwdd&Jboth near their origin and near their 

 termination. It is presumed that each fibril of which the axis- 

 cylinder is composed passes directly through from its point of 

 origin of the nerve-centres, to its final point of distribution, 

 without branching. It is difficult, however, with the instru- 

 ments in ordinary use, to see any distinct marks of fibrillation 

 in cross-sections of the axis-cylinder, and it is in them that we 

 should expect to see them best. The ideas of Ranvier are well 

 worthy of consideration, as he has given more form and solid- 

 ity to our conception of the intimate structure of a myelinic 

 nerve-fibre than any previous writer. According to him, each 

 section of nerve between the annular constrictions represents 

 an ultimate morphological element. It is, in fact, a tubular 

 cell, whose proper external portion (the membrane of the cell, 

 according to common phraseology) is the sheath of .Schwann, 

 while the myeline or medulla fills the interior, just as in adi- 

 pose tissue a globule of oil fills out and distends an ordinary 



