The sympathetic fibre, according to these writers, differs from the cerebro-spinal fibre in the following particulars ; it exhibits at its mar gin a single contour, instead of the double one which is so constant a feature of the cerebro spinal fibre, especially when examined some time after death; the distinction between a containing tube and the pulpy contents is not manifest ; the fibre has sometimes a greyish aspect, which the authors regard as independent of any admixture with material foreign to that of the nerve-fibre itself ; it is much smaller than tbe cerebro-spinal fibre, nearly one-half; in the cerebro-spinal as well as the sympathetic nerves both kinds of fibres may be found, but in the latter these peculiar fibres are enormously predominant, so that roths of their elements, or even a larger proportion, are composed of them ; in passing into neighbouring trunks they run as often cenuad as to the periphery. When the sympathetic fibres occur in cerebro-spinal nerves, they are collected into separate bun dles. The nervous branches which go to the involuntary muscles contain almost exclusively the smaller or sympathetic fibres. Mucous membranes are almost exclusively supplied by these fibres. The viscera of the chest and ab domen receive nerves which are made up almost exclusively of sympathetic fibres.
[These statements are quite at variance with the results of rny observations, as well as of those of Ilenle and Valentin.] The authors remark a considerable difference as regards the relation which these peculiar fibres bear to the cerebro-spinal centres in Frogs, AIammalia, Birds, and Fishes. In frogs the tine fibres originate in greatest part from the ganglions on the posterior roots of spinal nerves and from those of the sympa thetic. In Mammalia the brain and spinal cord are not the only sources of the sympathetic fibres. The ganglia also probably give off some. In birds the ganglion of the vagus is a pro bable source of sympathetic fibres ; and in fishes the great thickness of the branches of the vagus, which are very rich in the fine fibres com pared with the small size of its roots which are deficient in them, indicates that the ganglion of that nerve is a source of very numerous sympathetic fibres.
The anatomical statements of these writers would, if founded in fact, go far to confirm the opinions of those physiologists who uphold the inde pendence of the sympathetic system, and to prove the ganglia to be distinct centres of nervous influence. It is impossible to enter fiirther upon the discussion of this question at present, without introducing physiological ar guments. In a subsequent part of the article we shall retum to the subject.* Nerves of Invertebrato.—In those Inverte brata in which a definite arrangement of the nervous system has been made out, the same elements of the nervous matter are to be found as in the Vertebrata. The grey matter consists of globules with nuclei and nucleoli precisely like those of the human brain. From the gan glia the nerves radiate ; the nerve-tubes, which are very delicate and transparent in the recent state, contain a soft pulpy matter easily altered by re-agents. They are themselves collected into bundles which are surrounded by a clear transparent membrane, of the same kind as the sarcolemma of muscle, which accompa nies and surrounds the branches of the nerves. As the nerve-tubes separate from the primary mink into smaller fascicles, these sheaths bi furcate, so as to adapt themselves to the new branches. From the clear outline of the sheath, and the faintness and indistinctness of the margins of the nerve-tubes contained within it, this arrangement in the smaller nerves has very much the appearance of a bifurcation of the nerve-tubes themselves. There seems, however,
no reason to believe that the nerve-tubes of In vertebrata follow a different law from that which regulates their disposition in the Vertebrate series. It is likewise highly probable that the relations of these nerve-tubes to both periphery and centre are essentially the same as in Ver tebrata. Plexuses occur much more rarely, according to Valentin, in the nerves of Inver tebrata.
Of the development of nerve.—We can add nothing to the account given by Schwann of the developement of nerve. The following is quoted from Wagner's Physiology.
" The nerves appear to be formed after the same manner as the muscles, viz. by the fusion of a number of primary cells arranged in rows into a secondary cell. The pri mary nervous cell, however, has not yet been seen with perfect precision, by reason of the difficulty of distinguishing nervous cells whilst yet in their primary state, from the indifferent cells out of which entire org,ans are evolved. When first a nerve can be disunonished as such, it presents itself as a pale cord'with a longitu dinal fibrillation, and in this cord a multitude of nuclei are apparent. ( Fig. 335, a.) It is easy to detach individual filaments from a cord of this kind, as the figure just referred to shows, in the interior of which many nuclei are included, similar to those of the primitive muscular fasciculus, but at a greater distance from one another. The filaments are pale, granulated, and (as appears by their farther developement) hollow. At this period, as in muscle, a secondary deposit takes place upon the inner aspect of the cell-membrane of the secondary nervous cell. This secondary de posit is a fatty white-coloured substance, and it is through this that the nerve acquires its opacity ( Jig. 335, b). Superiorly the fibril is still pale; inferiorly, the deposition of the white substance has occurred, and its effect in rendering the fibril dark is obvious. With the advance of the secondary deposit, the fibrils become so thick, that the double outline of their parietes comes into view and they ac quire a tubular appearance (c). On the oc-. currence of this secondary deposit the nuclet of the cells are generally absorbed ; yet a few rnay still be found to remain for some time longer, when they are observed lying out wardly between the deposited substance and the cell-membrane, as in the muscles (c). The remaining cavity appears to be filled by a pretty consistent substance, the band of Remak, and discovered by him. In the adult a nerve, consequently, consists, 1st, of an outer pale thin cell-rnembrane--the membrane of the ori ginal constituent cells, which becomes visible, when the white substance is destroyed by de grees (d); 2d, of a white fatty substance de posited on the inner aspect of the cell-mem brane, and of greater or less thickness; 3d, of a substance, which is frequently firm or con sistent, included within the cells, Me band of Remak." From the ',description given in the foregoing pages, we have seen that the prevailing anato mical element of nerves is a tube composed of homogeneous membrane containing a soft pulpy matter, the true nervous substance, divisible into the white substance of Schwann and the band of Remak, and that through the medium of these nerve-tubes or fibres the grey matter of the central masses is brought into connection with the peripheral textures and organs.