All the fibres which are sent from the cerebro-spinal system to the sympathetic, through the medium of the communicating branches, are probably derived, according to Volkmann, from the posterior roots of the spinal nerves alone, and are not therefore possessed of motor properties. They hold the relation of centripetal or afferent fibres to the ganglia of the sympathetic. The motor properties of the sympathetic are therefore considered by him to be due entirely to the fibres which arise in the different ganglia. In regard to those movements which, as already stated, are excited in organs supplied with sympathetic nerves, by irritation of' the cen tral masses of the nervous system, Volkmann holds that the stimuli to contraction in these cases are not transmitted directly to the organs in which the contractions are mani fested, but are first conveyed by the -fibres in the rami communicantes to the ganglia of the sympathetic, where transference to the proper syinpathetic fibres takes place.
Thus, then, according to Volkmann, the motor properties of the sympathetic are en tirely due to the proper ganglionic fibres. The painful sensations which are sometimes felt in parts supplied by the sympathetic are due, not so rnuch to fibres of cerebro-spinal origin as to an altered condition of the gan glionic fibres, while the fibres which are sent to the sympathetic by the cerebro-spinal sys tem act as afferent or centripetal fibres to the different ganglionic centres, and by means of which a connection is established between the sympathetic and cerebro-spinal systems.
According to Valentin, again, both the motor and sensory properties of the sym pathetic are due entirely to cerebro-spinal fibres.
It is generally admitted that the sympa thetic receives fibres from the anterior as well as from the posterior roots of the cerebro spinal nerves. The number of these fibres must, moreover, be very considerable, espe cially in the higher animals ; it would seem probable, therefore, that the motor, and especially the sensory properties of the sym pathetic are in part due to these fibres. The experiments of Budge and Waller show, al most beyond a doubt, that, in the case of the iris at least, the motor fibres which pass to it through the medium of the sy mpathetic are derived from the spinal cord. The circum stance, however, that the organs supplied by the sympathetic cannot be influenced by the will, and in the normal condition are removed beyond the sphere of sensation, would seem to indicate that the conducting power of these fibres must be modified by the different ganglia through hich they pass in sorne such way as Volkmann supposes.
Are the ganglia to be regarded as centres of reflex action ? By Valentin*, Longetf, and others, they are denied this property.
Prochaska* seems to have attributed such properties to the ganglia, inasmuch as he ex plains the contraction of the heart by sup posing that the impressions which nre made upon the inner surface of the organ are trans mitted to the ganglia by means of sensory nerves, and are there transferred to motor nerve-fibres. Grainger t, in like manner, holds that the ganglia are centres of reflex action, and moreover that each ganglion pos sesses a distinct so-called excito-motory sys tem of' nerves. From what has been already stated, it will be observed that Volkmann also holds the view that, in the ganglia, trans ference of impression from one fibre to another takes place. From his earlier experiments t, however, he was led to conclude that such was not the case. He found, on applying a stimulus to the surface of the intestines in a newly-killed frog, that a contraction en sued which was not confined to the part which had been stimulated, but extended for a considerable distance on either side. After
destroying the spinal cord, and ag,ain applying the stimulus, he now found that the contrac tion produced was merely local, confining itself to the part irritated. The extended contrac tion first produced he believed to be due to reflex action, while the limited contraction in the second experiment he regarded as a mere stimulus movement. Frous the circumstance, moreover,that the former took place while the spinal cord yet remained, and the latter after it was destroyed, he concluded that it was thereby proved,—Ist, that the spinal cord is the centre in which the act of reflexion takes place in the movements of the intestine ; and, 2nd, that the ganglia are destitute of such power. Longet§ also states that it is only w hile the spinal cord remains that contrac tions extending over large portions of the intestine can be excited by local application of stimuli, the contraction so produced limit ing itself, after the spinal cord is destroyed, to the point irritated. As was shown by Henle, however, there can be no doubt that move ments inay be excited by application of stimulus to the surface of the intestine after the spinal cord is destroyed, which are as extended as those excited in the same way while it remains. The contractions produced by local stimuli are so similar both before and after the removal of the spinal cord as to leave no doubt that it can have no share therein. The only question is, whether the difference in character between the extended contractions and those which are limited to the point irritated are due to reflex action, or not. By Valentin and others, the extended contraction is explained in the same way as they endeavour to explain that of the heart, by supposing a particular arrangement of the muscular fibres, by means of w hich the con traction of one bundle acts as a stimulus to the neighbouring bundles, exciting them suc cessively to contraction. How far this is the case it is difficult to determine ; it seems, however, that the relation of the one bundle of muscular fibres to the neighbouring bundles in the intestine is not so different from what it is in the ordinary muscles as to explain the litnited contractions which take place in the latter, and the extended contraction of the former, upon the application of local stimuli. The opinion of Henle*, that they are of a reflex nature, the centres of reflexion being the grey matter of the sympathetic ganglia, seems, therefore, to be the more probable. Kiirschner also adopts the view that the gan glia are to be regarded as centres of reflex action. On repeating Mailer's experiment of irritating the solar ganglion with potash, he observed that the movements thereby pro duced in the intestines did not commence at a single point, but in several different coils of the intestine at one and the same time. This may, he says, be explained in either of two ways : the stimulus had either affected di rectly all the motor filaments, by which these different parts of the intestine are supplied, or only a few of them ; and from these few a transference took place, in the ganglion, to the others. The latter he believes to be the true explanation ; for he found it is quite the same, as regards the extent of the movements, whether the irritant is strongly or slightly applied, and whether a finely-pointed rod of potash or a broad surface of the same is em ployed.