According to Remakt, the fibres which have been described above as the gelatinous fibres or fibres of Remak, constitute the pe culiar organic or sympathetic nerve-fibre, all the tubular nerve-fibres being considered by him as belonging to the cerebro-spinal system. The fibres in question do not take their origin, according to him, from the cerebro-spinal centres, but arise from the ganglionic corpuscles contained in the different ganglia, and then run along with the tubular fibres sent to the sympathetic by the cerebral and spinal nerves. It has been disputed, however, whether the fibres described by Remak are entitled to the character of nerve-fibres, and are not rather to be regarded in the light of accessory struc tures which serve as a sheath to the true or tubular nerve-fibres. Valentin t, who adopts the latter view, states that the fibres in ques tion do not arise from the ganglionic cor puscles themselves, as was believed by Remak, but are continuous with the nucleated sub stance which forms the sheath or capsule of these bodies, and are thence prolonged upon the nerve-tubes, and are to be viewed as merely discharging the part of a protecting covering or envelope to the latter. They do not, according to Valentin, present the most distant resemblance to nerve-tubes, which could scarcely be the case were they in reality a mere variety of the same structures : in their microscopic character, on the other hand, they agree in every respect with certain forms of white fibrous tissue, especially at certain stages of its formation ; and they are entirely-wanting, or occur only in small num bers, in the main cord of the sympathetic, where the fibrous tissue is, as well as in many of the peripherical branches of the same, deficient. In the horse the fibres of Remak, which are present in the nerve-branches passing along the mesentery, were seen by Valentin to cease one or two feet from the point at which the nerves enter the intestine. Bidder* and Volkmann also appear to regard thern merely as a variety of areolar tissue, observing that their anatomical characters are so different from the known elements of the nervous system as to exclude them from the character of true nerve-fibres. In the mammalia, according to these authors, the areolar tissue which is interposed between the different organs is very abundant, and in this class of animals the fibres of Remak also abound. In birds, where the quantity of such areolar tissue is smaller, these fibres are not so nu merous ; and in the cold-blooded animals, where there is very little interposed areolar tissue, the fibres of Remak either fail al together, or they exist only in very small numbers. From this it would appear that they regard the fibres of Remak as holding the same relation to the tubular nerve-fibres that the areolar tissue holds to the different organs between which it is interposed. They also agree with Valentin in regard to the very marked resemblance between these fibres and white fibrous tissue at certain stages of its formation, and accordingly adopt the view that they are rather to be regarded as white fibrous tissue which has not reached its full development than true nerve-fibres. They find, moreover, as will be afterwards noticed, that the increase in point of thickness of the nerves leaving a ganglion over those which pass to it is due, not to the presence of a greater number of the fibres of Itemak, as might be expected were these true nerve fibres arising from the ganglionic corpuscles, but to an increased number of fine tubular fibres.
1(51liker A' likewise agrees with the authors already mentioned in the view that the fibres of Remak are not to be regarded as true nerve fibres, but rather as enveloping structures or sheaths to the tubular fibres. Ile believes, with Valentin, that they are not connected with the ganglionic corpuscle itself, but with its sheath, and are thence continued along such of the tubular fibres as arise from the corpuscles, forming for them a protecting enve lope or sheath. The capsules of the ganglionic corpuscles are, according to Kolliker, a species of fibrous tissue; and so also the fibres of Remak, which are continuous with them, must be regarded as partaking of the same nature. Again, whilst, on the one hand, these fibres are very abundant in the neighbourhood of the ganglia, in the finer branches, on the other hand, they are much fewer in number, and in the ultimate distribution of the nerve do not exist at all. Similar structures have also
been observed by him accompanying the very fine branches of some of the spinal nerves ; such, for example, as those going to the skin, while, at the same time, they are not to be found in the main or larger branches of the same.
The chief grounds, then, on which it is held that the fibres of Remak are to be re garded as enveloping structures, and not as true nerve-fibres, are, 1. the anatomical dif ferences between these and the true or tu bular nerve fibre ; 2. their resemblance to certain varieties of white fibrous tissue ; 3. their connection with the sheaths of the ganglionic corpuscles, and not with these bodies themselves; 4. their absence in the final distribution of the nerve ; 5. the in crease in the thickness of the nerves leaving a ganglion being due, not to an increased number of fibres of Remak, but of fine tubular fibres.
On the other hand, it has been stated that the anatomical difference between the tubular fibres and the fibres of Remak is not a suf ficient ground for believing that the latter are destitute of the properties of nerve-fibres. All the nerve-tubes in the embryo, even after it is considerably advanced in development, present much the same character as these fibres, and even after birth nuclei fray be occasionally found existing in them. Again, as noticed by Todd and Bowman, the nerve fibres in the olfactory nerve resemble the fibres of Remak in containing nuclei, and also in the want of double contour, as well as in their soft homogeneous appearance. When a nerve is divided, and a portion of it removed, the structure by which its continuity is re stored presents much the same appearance as the fibres of Remak, and this for some time after the part supplied by the nerves has, to a certain extent, regained its functions, showing that impressions may travel along structures not differing from the fibres in question. In reply to the second objection, it is stated that the difference between the fibres of Remak and white fibrous tissue is such as to preclude the notion of the one being a mere variety of the other. In the third place, it is said that it is by no means determined that the fibres of Remak are connected merely with the capsules of the ganglionic corpuscle, and, supposing they were so, that these also may be possessed of the properties of nerve-tissue.
As regards the relation between the gan glionic corpuscles and the fibres of Remak, the tubular nerve-fibres which leave the ganglia may not unfrequently be observed to have structures running along each side, which present the same characters as the fibres of Remak ; sometimes only a single row, at other times a double row, of nuclei are placed along each side of the tubular fibre, indicating one or two fibres of Renaak. These, on being traced inwards to the ganglionic corpuscle, are found to be distinctly continuous with the capsules of these bodies. When the ganglionic corpuscles are seen separately from one another it is found that these structures are connected only with such of the corpuscles as are still included in their capsules, those which are isolated from their capsules never having any such attached to them (see A, B, and c.fig.284.) In their general aspect, as well as in their relation towards reagents, the fibres of Remak correspond with the capsules of the corpuscles ; moreover K011iker has seen and distinctly figures the capsules of these bodies as directly continuous with the fibres of Remak. As regards their dissimilarity to white fibrous tissue, there can be little doubt that when characteristic specimens of either are examined the difference is sufficiently marked ; yet in some parts of the branches of the sympathetic in the higher animals it is difficult to limit the two species of tissue, and between the most characteristic of the fibres and many em bryonic tissues the most marked resemblance exists. In the ultimate distribution of the sympathetic there seem to be, as Kiilliker observes, none but fine tubular fibres present.