ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 577 



ammonia, or with alum, or with a sodic base. He also recommends 

 strongly Heidenhain's hematoxylin, (c) Hardening was best effected 

 by a solution of 1^-2^ per cent, double chromate of ammonia, (d) 

 Imbedding cannot be in any way satisfactorily accomplished by 

 paraffin, wax, or gum, but the celloidin method recommended by 

 Schiefferdecker was found most effective, (e) Dr. Gierke insists 

 especially on the necessity of having fresh material and thin sections, 

 and attributes many discrepancies of result to the absence of these 

 essential conditions. After noting briefly some of the current 

 descriptions of the histology of the central nervous system, and 

 emphasizing especially the incorrectness of the phrase connective 

 tissue, so often applied to the supporting substance, he selects as most 

 convenient Virchow's term, " neuroglia," including in that both the 

 amorphous ground-substance and the definite cellular elements, which 

 together form the matrix in which the nervous elements are im- 

 bedded. These two parts make up the whole neuroglia ; he denies 

 the existence of elastic fibres, connective-tissue fibrils, free nuclei, &c. 

 The only structures which occur are lymphoid cells which have 

 wandered in, or embryonic cells which have persisted unmodified. 



The matrix. — The ground-substance or amorphous matrix forms 

 along with the imbedded cellular elements (1) the outer and inner 

 enveloping mass of the central nervous system, (2) the matrix of the 

 grey substance, and (3) the stronger strands penetrating the white 

 substance. In the grey matter the ground-substance is abundant, 

 varying in different mammals in quantitative development apparently 

 in inverse proportion to the development of the nervous elements, 

 i. e. becoming less as the intelligence increases. It is uniform 

 throughout, homogeneous, structureless, transparent — a soft but firm, 

 not fluid, elastic albuminoid substance. The alleged existence of 

 imbedded molecules, on which so much stress has been laid by some, 

 e. g. Eindfleisch, rests on a misinterpretation of cross sections of 

 fibres, fibrils, and glia-cell processes ; the granular character of the 

 ground-substance described by even such accurate observers as Henle, 

 has a similar explanation — the granules belong to the glia-cells. 



Cellular elements of the neuroglia. — The neuroglia-cells, often 

 called spider-cells, form the greater part of the neuroglia, extending 

 through every portion of the matrix and forming with their long 

 uniting processes a supporting meshwork. They vary extremely in 

 size, form, nature of processes, and even in consistence, but exhibit 

 relatively constant characters in definite localities. 



Processes. — There are no glia-cells without processes, and though 

 cells with only one do rarely occur, there are usually many from 

 each cell. In the white substance the individual processes some- 

 times interlace before breaking up into extremely fine branches. 

 The marvellous network of processes is even narrower and finer in 

 the grey substance. 



Form and size of the cells. — The cells have of course no outer 

 envelope of any kind. The differences in size are extreme, varying 

 with the size of the nucleus, the development of processes, and 

 the degree of horny modification. Two prominent types, connected 



Ser. 2.— Vol. VI. 2 Q 



