5 Des Nerfs vaso-moteurs, Thèse de Concours, Paris, 1873.
6 Des Congestions actives, Paris, 1874.
The view already stated, that vaso-dilators act by inhibiting local ganglia which cause contraction, is now accepted, especially since it is found that such inhibitory activity is constantly displayed by other parts of the nervous system, and is competent to explain the facts. The active dilatation produced by the inhibition of the action of the local ganglia is therefore to be clearly distinguished from the passive dilatation caused by the cessation of normal tonic impulses sent to them from higher centres. The former is a positive active vaso-dilator phenomenon. The latter is a negative paralytic vaso-constrictor phenomenon. The former is more effective, the dilatation being greater in degree and more permanent than the latter, and resembles exactly the dilatation produced experimentally by exhaustion of the activity of the local ganglia by over-stimulation of the constrictors.7
7 Goltz, Arch. f. d. gesammt. Physiol., xi. 92.
An important point of contrast which has been established between vaso-constrictor and vaso-dilator impulses is that while the former are constant the latter are intermittent. Hence they cannot be regarded as opponents of one another. In a normal quiescent state vaso-constrictor energy is always being supplied to counteract the continued intravascular pressure ever renewed with the cardiac systole. The vaso-dilators are inactive. In an organ thrown into functional activity an increased flow of blood at once takes place, proportionate to the work being done by the organ. Such a functional hyperæmia might be produced either by a cessation of constrictor impulses or by an inhibition of their effects. It is by the latter means and through the vaso-dilators that it is produced, and it is probably the chief function of the vaso-dilator nerves to regulate the blood-supply in accordance with the need of a part. For this reason these nerves have been supposed to pass with the motor nerves to the muscles. As few organs exist without a possible use, it is probable that vaso-dilators pass to all parts of the body, as Vulpian asserts, though they have not been demonstrated in every organ or every part.
Like the vaso-constrictors, the vaso-dilators can be traced to the spinal cord, and their centres there are governed by a general centre in the medulla, which in turn may be affected by impulses from the cortex. A destructive lesion in any part of the vaso-dilator system does not produce as marked effects as one involving the vaso-constrictor system, since the symptoms of such a lesion will only appear when the vaso-dilators are called into play. When the vaso-dilator nerve to the submaxillary gland is cut, no change is observed until by some sapid substance put in the mouth its function should be excited, when the gland is no longer found to flush with blood as in the normal state. It is by means of the vaso-dilators that erectile organs become engorged with blood. Eckhard has shown the nervi erigentes of the penis to be vaso-dilator nerves. If they are destroyed, the organs will not respond to the wonted stimulus—a symptom which, however, would only be noticed at intervals. An irritative lesion of the vaso-dilator system may produce permanent congestion of an organ or part, but this seems to be rather more rare than a congestion from paralysis of the constrictors. It is seen in injuries of the peripheral nerves.
Origin of the Vaso-motor Nerves.—The exact course of the vaso-constrictor and vaso-dilator nerves has been traced from various parts into the central nervous system by the careful experiments of Dastre and Morat,8 and more recently by Gaskell.9 It is now established that they exist as separate nerves, although they often run together, and that they usually enter the spinal cord at different levels.
8 See Comptes rendus Soc. de Biologie, and Arch. de Physiol., 1879-84.
9 W. H. Gaskell, “On the Structure and Function of the Nerves which Innervate the Vascular System,” Journ. of Physiol., Jan., 1886.
If the various spinal nerves be cut singly from above downward, and the effects noted, and if the peripheral ends be irritated and the effects noted, and if with proper care the results be analyzed and compared, it will be found that the anatomical connections of the segments of the spinal cord with the sympathetic ganglia, which are so evident at each level, are not the ones by which physiological impulses pass out at that level. The vaso-constrictors of the head, which can be traced to the superior cervical ganglion, do not come from the upper cervical region of the cord, as might be supposed from the connections of that ganglion, but from the first three dorsal nerves. They reach the ganglion through the sympathetic cord in the neck, having traversed the inferior cervical ganglion on the way. There are vaso-constrictors in the cranial cavity which accompany the third, fifth, seventh, and twelfth nerves to the eye, face, and mouth. It is still undecided whether they originate in the cord and medulla, issue in the spinal accessory nerve, and with it enter the cranium (Gaskell), or reach those nerves by way of the carotid and vertebral plexus from the inferior cervical ganglion (Dastre and Morat). The vaso-constrictors of the arm, which can be traced to the inferior cervical and upper thoracic ganglia and to the thoracic sympathetic cord, are derived from the seven upper dorsal nerves. It is true that division of the roots of the brachial plexus causes a slight dilatation of the vessels of the arm, but this is so much increased when the dorsal nerve-roots are divided that it is evident that it is through them that the constrictor fibres chiefly pass. The vaso-constrictors of the leg, which can be traced into the second and third lumbar ganglia and lumbar sympathetic cord, are derived from the five lower dorsal and first lumbar nerves, and only join the crural and sciatic nerves after passing through the abdominal sympathetic. The thoracic viscera are probably supplied partly through the branches of the inferior cervical and thoracic ganglia, and partly through the pneumogastric, the latter statement being disputed by Gaskell. The abdominal viscera are supplied partly through the splanchnic nerves, which are made up of fibres issuing from the cord in the fifth to the twelfth dorsal nerves inclusive, and partly through the pneumogastric. Thus the dorsal region of the cord is the origin of the majority of vaso-constrictor fibres in the body.