NERVOUS TISSUE. 59 



delicate, apparently tubular, and contain a similar granular material to that 

 found within the corpuscle. Some of the processes terminate in fine transparent 

 fibres, which become lost among the other elements of the nervous tissue ; 

 others may be traced until, after losing their granular appearance, they become 

 continuous with an ordinary nerve-fibre. 



The white, otherwise called tubular or fibrous nervous substance, is found con- 

 stituting a great part of the brain and spinal cord, almost the whole of the 

 cerebro-spinal nerves, and a great part of the sympathetic. 



The tubes, when perfectly fresh, appear to be homogeneous, but they soon 

 separate into two parts, the white substance of Schwann and the axis-cylinder of 

 Purkinje, the whole being inclosed in a structureless membrane the tubular 

 membrane. 1 The white substance is regarded as being a fatty matter in a fluid 

 state, which isolates and protects the essential part of the nerve the axis-cylin- 

 der. The partial coagulation of this white substance which follows on cooling 

 gives the nerve-tube, when examined after death, a double contour the darker 

 part seen on the outside of the axis-cylinder being the white substance of 

 Schwann. In consequence of the extreme delicacy of the tubular membrane, 

 even slight pressure will often give nerve-tubes a varicose outline, and drops of 

 oil, from the transudation of the fatty matter, often form outside the tubular 

 membrane. This is, of course, promoted by the action of ether. 



The axis-cylinder constitutes about one-half or one-third of the nerve tube, 

 the white substance being greater in proportion in the nerves than in the cen- 

 tral organs. The axis-cylinder is perfectly transparent, and is therefore indis- 

 tinguishable in a perfectly fresh and natural state of the nerve. It is described 

 by Kolliker as being distinguished from the white substance by the fact that 

 though soft and flexible it is not fluid and viscid, but firm and elastic, some- 

 what like coagulated albumen, with which it appears for the most part also to 

 agree in its chemical characters. In appearance it is pale and homogeneous, or 

 more rarely finely granular or striated. 



Besides these nerve-fibres which consist of two distinct parts, others are 

 found in which only the axis-cylinder can be recognized, surrounded by its 

 medullary membrane, whilst there are again mere primitive fibrils found in 

 various parts, which are perfectly destitute of any visible structure, and only 

 recognized as nerves by their connection with ganglionic cells, or with obvious 

 nerve-tubes. 2 They display a great tendency to become varicose on manipula- 

 tion. The finely-striated appearance of those nerves, which consist only of 

 the axis-cylinder and its membranous investment, renders it probable that these 

 also are formed of an aggregation of the primitive fibrillas. 



Thus three different kinds of white nerve-fibres are described by recent 

 authorities viz., 1. Those which consist of the axis-cylinder, ensheathed in 

 the white substance of Schwann, the whole being invested by the tubular mem- 

 brane; 2. Those which consist of the axis-cylinder and medullary membrane 

 only; and 3. The primitive fibrils, of which it is believed that the axis-cylinder 

 of the more composite nerves is made up. 3 



Most of the nerves of the sympathetic system, and some of the cerebro-spinal 

 (see especially the description of the olfactory nerve), consist of a fourth de- 

 scription of nervous fibres, 4 which are called the gray or gelatinous nerve-fibres 



1 Dr. Beale describes and figures cases in which several fibres, some with, others without the 

 white substance, are inclosed in a common tubular membrane. See Phil. Trans., 1862. 



2 Schultze (Strieker's Handbuch, fig. 17, p. 109) represents these primitive fibrils, both in 

 their connection with ganglion-cells and with large nerves. See also below, Fig. 36. 



3 Schultze believes that the primitive fibrils are the essential elements of all nerves ; thus, 

 according to him, the essential difference between the gelatinous and the ordinary nerve fibrils 

 consists in the absence from the former of the white substance (medulla) of Schwann, while the 

 tubular membrane is present. The small nerve-fibres, on the other hand, described as primitive 

 fibrils or naked axis-cylinders, are either destitute of any investment, or surrounded merely by a 

 structureless basement membrane. 



4 The real nature of these fibres has been doubted by several authors. It seems better, how- 

 ever, and more consonant with the prevalent opinion, to describe them as truly nervous. 



