Different parts of the nerve appear to exhibit the property of sensibility in different degrees. In regard to this point, Valentin II gives the following as the results of his ex periments. 1st. The very grey branches which have passed through several ganglia do not, when the stimulus applied to them is slight, give rise to any signs which would indicate that pain was produced. Such branches are those which pass along the mesentery to the intestine ; strong stimuli, however, such as the application of a ligature or of chemical irritants, cause, when applied even to these branches, distinct signs of pain. 2nd. Irritation of the ganglia themselves is followed by signs of pain either immediately or after a short time. 3rd. The connecting cord of the sym pathetic is similarly circumstanced in regard to sensibility as the ganglia. 4th. The rami communicantes are as highly endowed with sensibility as the posterior roots of the neigh bouring spinal nerves. He found that section of a communicating branch did not destroy the sensibility of the corresponding ganglion : the main cord of the sympathetic must also be divided above and below the ganglion before this ensues. In the lumbar region Brachet* found that, when the communicating branches of three successive ganglia were divided, the central ganglion was deprived of its sensory properties. The greater the number of ganglia intervening between the point of the branches of the sympathetic, to which the irritant is applied, and the cerebro-spinal centres, the less distinctly, according to Valentin, does it give rise to signs of pain. Hence, the peri pheral branches are the least sensitive, while the rami communicantes are the most highly endowed with this property, the connecting or main cord of the sympathetic and ganglia being intermediate in this respect between these two. The nature of the stimulus ap plied has also an influence on the results produced : when the ganglia are merely prick ed, or their branches quickly divided, sometimes no sign of sensibility is evinced, whereas pressure, application of nitric acid or potash to the same parts give rise to distinct expres sions of pain.
In regard to the experiments which are made with a view to ascertain the sensory properties of this nerve, it is to be observed that in general it is only by application of very powerful stimuli that the phenomena of sensibility are elicited : they seem to act by producing a more or less abnormal condition in the part of the nerve to which they are ap plied, and hence the effects they produce may be regarded as belonging to the same category as the phenomena observed in diseased con ditions of the organs supplied by this nerve. In the normal or healthy condition the fibres of the sympathetic seem to be almost entirely destitute of the property- of communicating impressions to the sensorium. We do not know, as Volkmann observes, whether the organic muscles be at rest or in motion ; whether the glands secrete in larger or in smaller quantity ; whether the gall-bladder be full or empty. We are sensible of the impres sions made by the particles of food so long as they remain in the mouth, but, as soon as they reach the stomach or intestinal canal, we are no longer aware of their presence.
Motor properties.— That the sympathetic contains motor nerve fibres there can be no doubt ; irritation of its branches being followed by movements in the different muscular organs to which they are distributed. Thus irrita tion of the splanchnic nerves in the living animal, or immediately after death, is generally followed by more or less extensive contrac tions in the srnall intestine. Miller observed that the same result followed irritation of the semilunar ganglion : the same observation has also been made by Kfirschner.t Mechanical or chemical irritation, but especially galvanic stimulus applied to the filaments of the sym pathetic which pass to the heart, have the effect of accelerating the pulsations of that organ and of exciting it to renewed contrac tion after it has ceased beating. As move ments very frequently arise in organs supplied by the sympathetic, especially in the intestines, spontaneously, at least under the stimulus of the atmospheric air, it is sometimes difficult to determine whether the contractions which follow the application of a stimulus to any of the nerves be really caused by this, or whether they may not belong to those just mentioned. Frequently, however, the contraction follows the irritation so regularly as to leave no doubt that the two are connected ; if; moreover, the abdominal muscles in the cat or rabbit be removed, so that the thin and transparent peritoneum alone remains over the viscera, application of mechanical or chemical irritants to the splanchnic nerves in the thorax may still be observed to be followed, in many cases at least, by contractions in the intestine. In such experiments the air is prevented from acting upon the viscera by the intervening peritoneum, and in this way the fallacy above mentioned is less liable to occur.
It remains to consider the motor influence of the sympathetic in reference to the different muscular organs supplied by it.
Heart. —The heart, as has been already stated, derives its nerves from the sympathetic and pneumogastric. That the branches which
are supplied by the sympathetic exercise an influence over the movements of the heart, is shown by what has been already stated, that after it has ceased to beat, irritation of the branches which pass to it from the cervical ganglia will again excite it to contraction. Similar results frequently follow irritation of the ganglia them selves. When the galvanic stimulus is ap plied to the cardiac branches of an animal in which the heart has not yet ceased pulsating„ the effect is to augment the number of beats, and at the same time to increase their strength. In a rabbit in which the heart's action had ceased, Valentin * found that when the wires of the magneto-electric apparatus were applied, about of a millimetre distant from each other, upon the second thoracic ganglion of the right side, a very powerful contraction in the auricles immediately ensued : the ex periment was repeated several times, and with the same result. This also took place when the same stimulus was applied to the first thoracic ganglion. When, on the other hand, the wires were laid upon the aorta at the dis tance of ith of a millimetre from the heart. or upon the surface of the right ventricle, no effect was produced. He concludes, therefore, that the stimulus when applied to the nerves was, in this case, more effectual than when applied to the muscular fibres themselves. As regards the function of those filaments which are sent by the pneumogastric to the heart, E. H. Weber + believes that they exercise a re straining influence over the movements of the organ ; stimulus applied to the pneumogastric, according to his experiments, having the effect of retarding or altogether stopping its move ments. When the stimulus of the electro magnetic rotation apparatus was applied to the bulbus arteriosus in the frog's heart,—the part of the organ around which the fibres derived from the sympathetic are, according to him, chiefly distributed,—he found that the pulsations were increased in number as well as in strength. When, on the other hand, the same stimulus was applied to the upper portion of the inferior vena cava, where the filaments of the pneumogastric are mainly dis tributed, the effect produced was not an ac celeration but a retardation or stoppage of the heart's action. When a defined part of the vagus has been stimulated for some time continuously, the heart again begins to pulsate : when a portion of the nerve above this point is now stimulated, no effect is produced ; when, on the other hand, the stimulus is ap plied to a portion further down, nearer the heart, a cessation of its movements is again produced. The circumstance that the heart, after the stimulus has been applied to the pneumogastric for some time,again commences to beat, is attributed by Weber to the part of the nerve becoming exhausted, or losing its restraining influence, when the heart, being thus freed again, begins to pulsate. Budge*, however, attributes the cessation of the movements of the heart, produced by the application of galvanic stimulus to the pneu mogastric, not to any restraining power ex ercised by that nerve, but rather to a temporary exhaustion produced by the strength of the stimulus. In support of this view, he states that, although the movements of the iris chiefly depend upon the oculo-naotor nerve, yet Weber found, when the wires of the magneto-electric rotation apparatus were applied to this nerve within the cranium, that the pupil became dilated, remaining so for a considerable time after stimulus had been withdrawn, and then again slowly contracting. The effects thus produced upon the iris are, according to him, analogous to those pro duced upon the heart by application of the galvanic stimulus to the pneumogastric. More over, the nerves which are sent to the heart of the frog do not present the arrangement which Weber has described. No other fila ments than those which pass from the vagus are distributed to the heart of this animal, at least no others have been demonstrated. The vagus nerve becotnes united with the sympa thetic in the ganglion, which is situated about one line from the root of the pneumogastric ; and from this ganglion, which contains fibres of the vagus and sympathetic, springs, amongst other branches, a slender filament which is destined for the heart. This runs down wards on the inner aspect of the lungs, and passes along the veins to the auricles and ventricle, the former receiving the greater number of the nerve fibres. The branch in question contains fibres derived both from the sympathetic and also from the pneumo gastric. Again, such a restraining power must hold an opposite relation to the moving power in the normal condition ; the moving power would therefore express itself only in part, according as the other is in a latent' state or in a state of activity, and consequently sec tion of the vagus nerve ought, did it exert the restraining power in question, to be fol lowed by an acceleration in the moven:ents of the organ, which is not the case. Budge, therefore, seems to regard the fibres which are sent to the heart in the frog by the pneu mogastric, as possessed of motor and sensory properties.