It has already been stated that groups of cells exist in the gray part of the spinal cord, which, according to the class of animal, have more or less direct influence upon the muscles in the coats of the vessels. Thus, in a frog whose brain and medulla have been destroyed, in some hours the vessels of the web regain a considerable degree of constriction, which is again lost if the cord be destroyed. In the dog the vessels of the hinder limb recover their tone more or less perfectly in a few days after the spinal cord has been cut in the dorsal region, although just after the section they are widely dilated from the paralysis of their muscular coats. In a few days the cells of the cord can learn to accomplish, of their own accord, work which they have been in the habit of doing, only under the direction of the higher centre. From this we conclude that though the cord contains local vasomotor centres distributed throughout its gray matter, these are normally under the control of the vasomotor centre in the medulla, and this centre is really the chief station from which impulses destined to affect all the blood vessels must emanate.
This arrangement is quite comparable with that by which the ordinary muscles are made to contract. When the will causes a muscular contraction, the impulse starting from the cerebral cortex does not travel directly to the muscle, but it passes from the brain to certain cells in the cord and thence to the muscles. In fact, to these spinal agents the ultimate arrangement and coordination of the act is confided. So, also, the chief vasomotor centre in the medulla executes its function through the medium of numerous under centres placed at various stations along the 'cord.
The vasomotor centres - like nearly all other controlling groups of ganglion cells - may be considered as composed of two parts antagonistic one to the other, viz., a constricting and dilating centre, the impulses from which travel by separate nerve channels. The constricting impulses are mainly distributed by the sympathetic nerve, while the dilating impulses accompany those which are employed in calling forth the ordinary function of the part in question.
From what has been said as to the wide distribution of centres influencing the blood vessels, an attempt to localize exactly the position of the medullary vasomotor cells is not satisfactory. In lower animals - frogs - the cells are evenly diffused throughout the medulla and cord. In man the localization is difficult to demonstrate, though we have reasons for thinking it much more definitely circumscribed. In the rabbit it has been localized to the floor of the fourth ventricle in the immediate neighborhood of the respiratory and cardiac centres. From this the nerves pass by the cord to the spinal roots, by which they reach the sympathetic.
The vasomotor centre exerts a tonic or continuing action on the vessels, holding them in a state of partial constriction or tone. In this it may be said to have an automatic action. Although this tonic state of activity of the centre may be called automatic, it is really under the control of many reflex influences, which constantly vary the general tone, or effect local changes in the degree of constriction of this or that vascular area. Among the most striking afferent regulating impulses are those arriving from the heart, the digestive organs and the skin. In some animals, a special nerve - the depressor - has been discovered, which, passing from the heart to the medulla, keeps the vasomotor centre informed as to the degree of tension, etc., of the heart cavities. When the heart becomes over-full, impulses pass from it, and check the tonic power of the centre, so as to reduce the arterial pressure against which the ventricle has to act. Electric stimulation of this nerve causes a remarkable fall in the general blood pressure. The vasomotor centres regulate the distribution of blood to the viscera and skin, according to the condition of activity of these parts as described in another chapter (xxxi).