Thus Remak and others describe three dis tinct parts in the nerve fibre :-1, the outer in vesting membrane, tubular membrane; 2, an inner layer of membmne (the white substance of Schwann) lying immediately within the first; 3, a central substance of nervous matter, called flattened band by Remak, and supposed by him to consist of several filaments, or the axis cylinder of Rosenthal and Purkinje.
It is evident that the contained matter of the nerve-tube is extremely soft: it yields under very slight pressure, and may be readily made to pass from one part of the tube to another. W lien pressed out of the nerve tube, it is apt to assume the appearance and form of globules varying in shape and size, which are easily distinguished from the true nervous globules by the absence of nucleus. Firm pressure will also completely empty the tubular membrane, and thus afford us a good opportunity of examining its struc ture, which has always appeared to present the same homogeneousness as the sarculemma of muscles to which we have compared it. Some observers, however, admit a complexity of struc ture in this tubular membrane; an appearance of longitudinal fibres has been noticed by Va lentin and Rosenthal, and the former describes a fibrous arrangement, as of oblique fibres wind ing in opposite directions, surrounding the tube.* The addition of water causes the contents of the tubular. membrane to separate from the inner surface of the tube, owing to a shrivelling or coagulation which it excites in the nervous pulp. Alcohol produces a similar effect, but occasions a more perfect coag,ulation of the soft nervous matter ; and it is particularly worthy of observation, that the complication of structure remarked by various observers and above de scribed, in the tubular membrane as well as in its contents, is never seen in the perfectly fresh nerve, but is always rendered visible by keeping or by the influence of various re-agents. And this fact may well excite a doubt a.s to the re ality of the complex structure of the nerve tube, as descnbed in the preceding parag,raphs.
In a word, the real structure of the primitive nerve fibre appears to be a tube composed of homogeneous membrane, containing a delicate, soft, pulpy, semi-fluid and transparent medulla or nervous substance, which is readily disturbed by manipulation, and altered by the addition of the simplest substances--even water. The tubes
when quite fresh are perfectly cylindrical ; but pressure or separation alters them in shape like wise, probably by disturbing the position of the nervous matter, pushing more than is natural into one part, and consequently diminishing the bulk of the contents of another part : the latter will consequently collapse, and the former become enlarged, distended, and even varicose. (Fig. 330.) The margins of nerve tubes that have been separated, for this reason constantly appear wavy, and at other times distinct swellings or enlargements form in the course of the fibre, separated by constricted portions. These swellings sometimes occupy one side of the tube only : in shape they are globular or ovoidal, and more frequently involve the whole tube; they exist at irregular intervals from each other, and are extremely variable in shape and size. Two conditions appear to favour the production of this varicose state of the nerve-tube,—namely, a feeble power of re sistance in the tubular membrane, and, se condly, perhaps, a semi-fluid state of the con tained nervous pulp; and hence we find that some nerve-tubes are much more prone to become varicose than others. In the nerves of pure sense the tubes are very delicate in structure and very apt to exhibit this change of form, and in the brain and spinal cord they exhibit the same tendency. Ehrenberg sup posed formerly that these varicosities were natural and existed during life, and that they afforded a valuable morphological character of the nerves of pure sense and the cerebro-spinal centres. But many circumstances favour the opinion that the varicosities are accidental ; thus, the very irregularities above noticed in their shape, size, form, and number on a single tube are not likely to occur in the natural state. Moreover, in a piece of the brain or spinal cord not much pressed nor torn, the cylinders may be distinctly seen : even in the manipulated specimens the varicose tubes form only a small portion of the whole. And in those nerves whose fibres are not prone to become varicose, such as muscular nerves, they may be made so by firm pressure and violence in manipulation. In the nerve-tubes of young animals, in whom the tissues are more tender and contain more abundant water, these changes are also very apt to take place.