CHAP. X.] THE NERVOUS TISSUES. 421 



describe all the various forms which nerve-cells present ; and mention 

 will merely be made of certain observations which refer to the most 

 marked and readily investigated of these structures. 



The processes which are given off by many nerve-cells, as by the 

 nerve-cells of the grey matter of the spinal cord, are numerous, and 

 such cells are often spoken of as multipolar. These processes are 

 extremely fragile, but under favourable circumstances they may be 

 observed to give off a number of fine branches. In addition, it has 

 been maintained (Deiters) that one process which is usually distin- 

 guished from the rest by its much greater thickness and length 

 becomes continuous with the so-called axis cylinder of a medullated 

 nerve-fibre ; such a process would, on this view, place the nerve-cell 

 in direct communication with a nerve-fibre. 



The other finely ramifying processes anastomose with similar 

 processes from other nerve-cells, giving rise to a reticulum from which 

 probably arise the axis cylinders of other nerve-fibres. Such a fine 

 reticulum can readily be seen in the grey matter, though it is some- 

 times difficult to establish which parts of it are purely nervous and 

 which belong to the connective tissue. Nerve-cells may o-r may not 

 have a sheath or investment. 



We are acquainted with very few facts relating to the micro- 

 chemistry of the nerve-cells ; they are doubtless in the main proto- 

 plasmic in composition, and are therefore specially rich in proteid 

 substances. From the analysis of the grey matter as compared with 

 the white, we conclude that the nerve-cells are comparatively poor in 

 the complex phosphorized constituents, and in other bodies, such for 

 instance as cholesterin, which are found in large quantities in nervous 

 organs as a whole. From the abundant supply of blood to the grey 

 matter as compared with the white we may assume that respiratory 

 exchanges go on much more actively in nerve-cells than in nerve- 

 fibres, a conclusion strongly borne out by the previously mentioned 

 discovery of haemoglobin in the nerve- cells of the ganglia of Aphro- 

 dite aculeata* 



Nerve-fibres, 



We may conveniently divide nerve-fibres into the two classes of 

 (1) medullated, (2) non-medullated nerve-fibres ; the former are much 

 the more abundant. 



1. Medullated nerve-fibres. When examined in its yet living 

 condition the medullated nerve-iibre presents the appearance of a 

 perfectly pellucid homogeneous structure which might at first be 

 thought to be a tube with transparent walls, containing a transparent 

 liquid ; a careful examination of all facts causes one, however, to 

 reject this view without hesitation. 



At death the nerve-fibre undergoes changes in its physical consti- 

 tution, and it then can be shewn to present (1) a highly transparent 

 membranous envelope, termed the neurilemma, in which, or beneath 

 which, are oval flattened nuclei, (2) a central structure, the axis cylin- 



