A system of practical medicine. By American authors. Vol. 5

by Unknown

The origin of the vaso-dilators is also to be traced to the dorsal cord. The vaso-dilators of the bucco-facial region come from the second to the fifth dorsal nerves, whence they pass to the first thoracic ganglion, and thence by the annulus of Vieussens into the cervical sympathetic cord. Those of the eye, head, and ear come from the same ganglion, but have their spinal origin in the eighth cervical and first dorsal nerves. Those of the arm are traced into the upper thoracic sympathetic cord, which they reach from the five upper dorsal and last cervical nerves. Those of the leg are traced to the first and second lumbar ganglia and the lumbar sympathetic cord, which they reach from all the dorsal nerves from the sixth downward. Gaskell holds, however, that the vaso-dilators of the extremities pass out of the cord in the cervical and lumbar plexuses and accompany the cerebro-spinal nerves. The vaso-dilators of the thorax and abdomen are supposed to pass in the pulmonary plexus and splanchnic nerves, but this is not yet fully determined.

Gaskell¹⁰ believes that vaso-motor nerves can be distinguished from motor and sensory nerves in the spinal nerve-roots by the smallness of their calibre. He finds such fine fibres only in the spinal nerve-roots between the second dorsal and second lumbar segments of the cord (in the dog), and in the three upper roots of the spinal accessory nerve. According to his account, the vaso-constrictors issue from the spinal cord in both anterior and posterior nerve-roots as medullated fibres, and pass to the sympathetic ganglia lying on the vertebræ (proximal or lateral ganglia); there they lose their medullary sheath, and either end in cells whence new fibres issue, or more probably pass directly onward as non-medullated fibres, having a connection with the unipolar cells of the ganglia only for purposes of nutrition. The number of fibres issuing from any one ganglion is much greater than the number entering it from the cord; hence it is supposed that each medullated fibre splits up into a group of non-medullated fibres; which is possible, as the researches of Ranvier have shown that each axis-cylinder is made up of numerous fibrils. Leaving these ganglia, the nerves pass either to the second series of ganglia (distal or collateral ganglia), whence they issue in plexuses to enter the vessels, or to the vessels directly, where they divide in plexuses. It is in the meshes of the plexus that the local ganglia of the vessel-walls are found. The vaso-dilators are thought to differ from the vaso-constrictors in passing directly to the distal ganglia as medullated fibres, not being connected in any way with the proximal ganglia. Gaskell makes no statement regarding their limits of origin from the cord, except to state that the nervi erigentes issue with the sacral nerves. He agrees with the view that the vaso-dilators act as inhibitory nerves upon the local ganglia.

¹⁰ Journal of Physiology, Jan., 1886.

Vaso-motor Reflexes.—Thus far, reference has been made only to fibres whose direction of transmission is centrifugal, and whose exit from the spinal cord is by the anterior nerve-roots. There are other fibres, however, through which centripetal impulses pass, and these enter the spinal cord with the posterior nerve-roots. The function of these fibres is to transmit sensory impulses inward to reflex centres, and thus set in action motor mechanisms of a vaso-constrictor or dilator kind whose effects are produced at the periphery. There are, therefore, vaso-motor reflexes, as well as skin and tendon reflexes, whose centres are in the spinal cord. These reflex acts may be excited by impulses reaching the centre not only through the vaso-motor centripetal nerves, but also through the sensory nerves of the cerebro-spinal system. The effect of changes of temperature on the circulation in the skin (if the right hand be plunged in cold water there is a fall of temperature in the left hand), the effect of pain upon the color of the face and the size of the pupil, the red cheek on the affected side in pneumonia, the occurrence of glycosuria during sciatica,—are all instances of such reflex acts. Many vaso-motor affections are produced by irritation causing reflex effects at a distance from the seat of irritation—a fact always to be kept in mind. The utility of counter-irritation to the surface in diseases of the internal organs is explained by supposing that vascular changes are produced in those organs through reflex mechanisms set in action by the local irritation. While some of these reflexes may have their central mechanism in the local ganglia, it is probable that the majority are to be traced to the spinal cord. It is believed that the sympathetic ganglia are not the seat of reflex centres.

FIG. 57.

Diagram to represent the Mode of Action of Counter-irritants applied to the Chest (Lauder Brunton). The irritation of the afferent nerves by the blister on the chest wall gives rise to a vaso-constrictor reflex in the vessels of the lung.

Since the vaso-motor nerves are connected almost exclusively with the dorsal portion of the spinal cord, it is very natural to conclude that the vaso-motor reflex centres are situated in this region; and the hypothesis has been advanced by Jacubovitch, and strongly urged by Gaskell,¹¹ that the cells of the vesicular columns of Clarke, which are peculiar to this region, are the seat of these reflex mechanisms. This hypothesis gains some support from the pathology of syringo-myelia. In this disease the gray matter surrounding the central canal and the vesicular columns are destroyed. The characteristic symptoms are vaso-motor and trophic disturbances, consisting of changes in the vascular tone, changes of local temperature, and various eruptions, in some cases going on to ulceration in the skin and mucous membranes. It is, however, undecided whether the vaso-motor centres of the cord are limited to the columns of Clarke, or are situated in the gray matter surrounding the central canal, since both these parts are destroyed in this disease.¹² That they are not located in the anterior or posterior gray cornua is determined by the fact that diseases limited exclusively to these areas do not cause vaso-motor disturbances. The situation of the various reflex centres for the various parts of the body is at different levels of the cord, as has been determined by the experiments already cited to establish the level of origin of the vaso-motor nerves. The exact location of the vaso-constrictor and vaso-dilator reflex centres for definite parts is yet to be ascertained.

¹¹ Loc. cit.