2. In regard to the structures No. 2. which have been mentioned as occurring in the sym pathetic, Remakt, their discoverer, describes them as being destitute of a sheath, naked, transparent, almost gelatinous, as presenting a number of longitudinal streaks, and as break ing up into delicate fibrils, which in their course present oval dilatations or swellings, and have moreover a number of round or oval cor puscles arranged upon them at intervals. Apparently, the same structures have been described by Henle under the name of " gela tinous fibres." According to him they are flat homogeneous fibres, measuring from the 0'002' to the 0-003'" in diameter, and cha racterised by the presence of numerous nuclei, some round, others oval, their long diameter being directed in the longitudinal axis of the fibre. They are dissolved by acetic acid, the nucleus becoming at the same time more dis tinct. The fibres in question are well seen in the branches of the sympathetic which go to the spleen or kidney in the sheep or ox, as well as in the trunk of the sympathetic nerve itself. In the former situations they appear as a more or less transparent, slightly granular, pale mass, and marked by indistinct longitudinal lines into fibres presenting a diameter of about „1,7-,th to Tri,-,1 —„th of an inch, and characterised by the presence of round, oval, or elongated nuclei. On the addition of acetic acid they swell out, becoming perfectly transparent and in distinct, while at the same time the nuclei are brought more clearly into view. When treated with tartaric or citric acid, the effect produced upon them is much the same : solution of soda also causes them to swell out and be come indistinct, the nuclei being at the same time also rendered more or less indistinct. The nuclei generally measure about the 1-j„,th to the of an inch in length, and about al,,,th to the 5th of an inch in breadth, presenting the same characters and behaving towards reagents in [the same manner as the nuclei occurring in most other tissues. They are much softer than the tubular fibres, and are not easily separated from one another. In some parts of the peripheral branches of the sympathetic these fibres present a much smaller diameter, measuring about -dr—, oth of an inch, are finer and distinguished with difficulty from the white fibrous tissue present. In the nerve they are placed parallel to one another, and are seen, when the preparation is pressed between the glasses, running along each side of the tubular fibres, which latter seem to be im bedded amongst them. They differ from the tubular nerve-fibres in their flattened ap pearance, want of distinct margin, and in the effects produced upon them by reagents, but are especially characterised by the presence of their nuclei. They are most abundant in the more grey-looking branches of the sym pathetic, and seem to be the cause to which this appearance is chiefly owing. Sometimes when one of the smaller filaments of the sympathetic is examined, it seems to be en tirely composed of these fibres, no tubular nerve fibres being at first seen ; solution of soda, however, which, as has been stated, renders the gelatinous fibres transparent, brings into view more or fewer fine tubular fibres.
There can be no doubt that in many parts of the sympathetic, especially in the branches distributed to the arteries, the fibres of Remak, or gelatinous fibres,make up the greater portion of their constituents, the tubular nerve fibres existing only in comparatively small numbers Sometimes more or less grey and white brindles of fibres may be seen running along side each other : such an arrangement is not unfrequently seen in the branches of com munication between the sympathetic and the spinal nerves. In such cases, while the white
chiefly or entirely consists of tubular nerve fibres, the grey contains a large number of gelatinous fibres, and always in addition to these more or fewer fine tubular fibres.
The gelatinous fibres are present in dif ferent proportion in different parts of the sympathetic : they appear to be more abundant in the neighbourhood of the ganglia than in other parts and in the larger peripherical branches they also exist in considerable pro portion, but in the final distribution of these they either do riot exist at all or only in small number. They appear to be more abundant in the sy mpathetic of the higher animals than in that of the lower vertebrata. In mammals and birds they exist in considerable quantity: in amphibia, according to Kiilliker, they are present but only in small proportion. In some fish, as in the common Ray, the sympa thetic ganglia and branches contain a very large proportion of structures which agree with the fibres of Remak in some respects, such as in their relation to the ganglionic corpuscles and tubular nerve fibres, as well as by the presence of a number of small oval nuclei ; they differ from them, however, in not being so much affected by acetic acid and in being firmer : the number of tubular fibres occurring in the sympathetic is very small compared with the number of these struc tures.
3. The quantity of white fibrous tissue present in the sympathetic trunk and branches is generally considecable; the fibres are arranged in the longitudinal direction for the most part ; other fibres, which from their relation to reagents appear to belong to the yellow elastic tissue, encircle the nerve, binding together, as it were, its constituents. After addition of soda or acetic acid the circular fibres are well seen ; at the parts where they occur there is frequently observed a distinct con striction, the nerve being swollen out above and below by the reagent applied.
With respect to the nerve fibres occurring in the sympathetic, many of them present undoubtedly the same characters as those occurring in the nerves of the cerebro-spinal system. It has been maintained that the sympathetic also contains fibres which differ in their anatomical characters from the fibres of which the latter class of nerves are com posed, and which have been termed organic or vegetative nerve fibres.
Ehrenberg appears to have regarded the fine varicose tubular fibres which are present in the sympathetic, as constituting the peculiar organic nerve fibres. According to Purkinje*, the ganglionic nerve-fibres are much finer than those belonging to the cerebro-spinal system or animal fibres. He describes the latter as containing two substances,—an outer, which runs in the form of a tube through the elementary fibre immediately within its sheath, and an inner, which occupies the hollow in terior of the former. The tubular sheaths of the elementary fibres of the ganglionic system contain, on the other hand, no double substance ; their contents are homogeneous, and appear to correspond to the axis cy linder or central portion of the animal nerve fibres ; their sheaths are much stronger than those of the nerve-fibres of the cerebro spinal nerves, and resist mechanical influ ences in a high degree. In the feetus the anitnal nerve-fibres cannot, according to Purkinje, be distinguished at a certain stage of their formation from those which are cha racteristic of the sympathetic in the full-grown animal ; and hence he regarded the latter as a less highly developed stage of the former. Pappenheirn also appears to have recognised the fibres described by Purkinje, as consti tuting the peculiar fibres of the sympathetic system.