It has been already stated, that the nerve fibres which compose the posterior root of the spinal nerves in the ray, &c., have all, according to Wagner, ganglionic corpuscles developed upon them. He concludes from this, that all sensory fibres are so constituted, and that we have thus a good mark by which a sensory nerve-fibre may be distinguished from one possessed of rnotor properties. To this view, however, it is objected by KaHiker, that in the higher animals at least, so far is it from being the case, that all the fibres in the posterior roots of the spinal nerves are provided with these structures, that not one of the fibres proceeding from the spinal cord enters the ganglion at all, the nerve fibres connected with the ganglion be ing fibres which arise in it and run peripheri cally, not one of them passing in the opposite direction towards the spinal cord. In ex amining the spinal ganglia of the mouse, after addition, as Kfilliker directs, of dilute solu tion of soda, I have often had no difficulty in observing, that a great portion at least of the fibres in the posterior root run past the ganglion without forming any connection with its corpuscles, and, moreover, that the fibres of the ganglion appear to be directed periphe rically, as he states.* tomists, the sympathetic was described as a continuation of the fifth and sixth cranial nerves, reinforced by fibres sent to it from the different-cerebro spinal nerves along its course. The communicating branch between the carotid plexus and the sixth nerve, and the deep or carotid branch of the vidian, were regarded as the roots by which the nerve commenced, while the different branches passing between it and the other cerebral and spinal nerves, were believed also to be entirely composed of fibres sent by the latter to the sympathetic.
According to Bichat, the sympathetic is an independent system of nerves ; the cords which pass between it and the cerebral and spinal nerves are not entirely composed of fibres sent to the sympathetic, but are partly branches transmitted by it to these nerves. The observations of Petit and Fontana* had already shown that the communication be tween the sixth nerve and the sympathetic did not consist of fibres sent by the former to the latter, inasmuch as the sixth nerve was found to be thicker beyond the point of junction with the filament than before.
In 1827 Retziusf showed that in the tri facial nerve in the horse there was present a gray fasciculus of fibres distinct from the white, and which seemed to take its origin in the ganglion. Somewhat similar observations were made by Varrentrap and Milner on the branches of the trigeminus, and by Giltay on the glosso-pharyngeal, vagus, and superior spinal nerves of the fish, &c. It was after wards noticed by Remak that the gray por tions of the communicating branches con sisted of fibres which were sent by the sympathetic to the cerebro-spinal nerves to be distributed peripherically with them. On microscopic examination it was found by him that the sympathetic contained a large num ber of fibres presenting a peculiar structure : these he regarded as the proper organic or symi)athetic nerve-fibres, and believed that while the sympathetic derived from the brain and spinal cord all the tubular fibres con tained in it, the grayer portions of the rami communicantes were composed of organic or sympathetic fibres, which were sent by the sympathetic to the cerebro-spinal nerves, to be distributed peripherically with them. The same view was also adopted by Miller and others. Valentin, as has been already stated, rejecting the fibres of Remak as being destitute of the properties of nerve-fibres, believed that the rami communicantes con sisted entirely of fibres sent by the brain and spinal cord to the sympathetic. Volkmann and Bidder, though agreeing with Valentin in regard to the fibres of Remak, still main tained the opinion, that the rand communi cantes are of a compound nature, containing fibres which are sent to the sympathetic from the cerebro-spinal nerves, and also others which are sent to the latter by the sympa thetic, and which belong to the fine variety of tubular fibres already described as probably arising in part from the ganglionic corpuscles.
On examining the connection between the sympathetic and cerebro-spinal nerves in the frog, they find that all the anterior branches of the spinal nerves communicate with the sympathetic. The filament of communication with the first spinal nerve at its entrance into it divides into two portions, one of which proceeds towards the spinal cord, the other towards the periphery : when it consisted of two portions, the one was directed towards the centre, the other ran peripherically. Con nected with the second spinal nerve they found several communicating filaments, the smaller portion of the fibres of which ran towards the centre, while the larger portion was directed towards the periphery. The fibres connected with the third nerve also ran in both directions, the chief portion, however, towards the centre. The fourth communicating branch sent its fibres both towards the centre, and also towards the periphery, the portion running centrally, however, being much more considerable than that running towards the periphery. So also in regard to the fifth ; the portion, how ever, directed towards the centre did not exceed that passing peripherically so much as in the former. Sometimes they found that the central and peripherical portions were about equal. The sixth communicating branch sent about an equal portion of its fibres in either direction. In regard to the seventh, they found that by far the greater portion was directed peripherically, while only a very small bundle took the direction of the centre. Between the eighth nerve and the sympa thetic there are frequently two communicating filaments : their fibres are directed almost ex clusively towards the periphery, only a very small portion being directed towards the centre ; and sometimes even this is wanting. Between the ninth nerve and sympathetic there are commonly two, often also three, filaments of communication with the s3mpathetic; and in one case they found as many as six : the course of the fibres here is similar to what it is in the eighth ; perhaps, however, the portion sent inwards towards the centre is even smaller, and not unfrequently fails alto gether. The communication with the tenth nerve they found was not constant : some times three communicating filaments were observed ; at others no communication ap peared to exist. When present, they always ran almost exclusively in the direction of the periphery. Thus, then, of the rami communi cantes in the frog there appear to be none which consist of fibres entirely derived from the spinal cord, while, on the other hand, some of these consist ahnost exclusively of fibres which run towards the periphery, and which therefore must be regarded as exclusively consisting of fibres which are sent by the sympathetic to the spinal nerves. The five up per spinal nerves give to the sympathetic in the frog more fibres than they receive from it, while, on the contrary, the five lower receive from the sympathetic more fibres than they send to it. As regards the communicating branches between the sympathetic and cere bral nerves, they also regard it as probable that the greater number of the fibres in the conomunicating branches run peripherically. In the fish and bird they also found that the fibres of the communicating branches were di rected partly towards the centre and partly to wards the periphery. In small animals belong ing to the class mammalia, such as the rat and mole, as well as in small dogs and cats, they found, on examining the communicating branches with the microscope as before, that the fibres passed both inwards towards the centre. and also outwards towards the peri phery, and that the latter in many cases ex ceeded the former.