level of blood-pressure is maintained through the channels of secretion by varying amounts of the active substances of these two glands. He explains the blood-pressure variation in myxedema and early Graves disease upon this basis. That these substances are alone in control of the field is very doubtful, as recently Voegtlin and Macht1 claim to have discovered a new vasoconstrictor substance from the blood of the adrenal cortex, whose physiologic action is somewhat similar to that of digitalis, while chemically it is akin to lipoid cholesterin. While these several theories may be in part reconciled to our present knowledge of the relations of blood-pressure in health and disease, they must not be accepted as conclusively proven. For we have no proof that the substances extracted from these glands ever normally enter the blood-stream in a chemical form that we are able to isolate them. 4. The Vasomotor Nervous Mechanism Controlling Blood-pressure.-The Maintenance of Tonus.-The condition of tonus may be defined as, a state of semicontraction of the several coats of the arterial walls, which is maintained through the activity of a complicated vasomotor mechanism. This condition of tonus is most important, since changes in the force and frequency in the heart's action, and variations in the total amount of blood in the body, or in the supply to any organ or part, would be at the mercy of every other part, unless controlled by the automatic contraction and dilatation of the vessels in other parts of the body. Such a mechanism is essential to the proper distribution of blood to all regions at all 1Carl Voegtlin and David I. Macht, Jour. A. M. A., December 13, 1913. times. This condition in any individual part is known as local vascular tone, and is under the control of a system of nerve ganglia with subservient fibers, which are found in the middle coat of all arteries. The energy expended by these ganglia is manifested by a constant moderate contraction of the circular muscular coat of the artery-a contraction which is constantly opposed by the dilating force (lateral blood-pressure) within the vessel. An exact equipoise between these two forces never occurs, since each factor varies constantly; but in a state of health, neither one ever becomes permanently excessive. Considerable variations, however, in the local vascular tone are frequently observed. Thus, each organ is influenced to a certain degree by every other, since an increase in blood in one part must necessarily involve a decrease in other parts, the total amount of blood in the vessels remaining constant. Those of the splanchnic area are chiefly concerned in maintaining this balance, since, if local vasomotor control of these regions were, for any reason, abolished, the arteries and arterioles would dilate sufficiently to contain practically all the blood in the body. The result would be a condition similar to that following a massive hemorrhage. Alterations in the heart's action are felt more quickly in some organs than in others and, thus the general bloodpressure for cardiac reasons may be varied sufficiently to cause secondary disturbances in local vascular tone.1 The nervous mechanism which controls local vascular tone is a complex one, consisting not only of a set of local ganglia in relation to the vessels and connected with the 1 C. J. Wiggers, Jour. Exp. Med., January, 1914, xix, p. 1. larger sympathetic ganglia; but also of centers in the spinal cord, which are in turn connected with higher centers in the brain. The brain centers in turn are complex, consisting of an automatic mechanism in the medulla, regulating the action of all the subordinate parts below it, and of a series of cortical centers whose function it is to stimulate or inhibit the medullary mechanism.1 Vasoconstrictors.-These facts seem to warrant the conclusion that the energy expended by the local ganglia in holding the vessels in a state of constant moderate contraction, is derived from the central nervous system, primarily from the automatic center in the medulla, which in turn is reinforced by each of the secondary centers in the spinal cord and sympathetic ganglia; and also that, while the medullary centers control the entire body, the cord and sympathetic centers control only these parts with which they are especially related. Any injury or interruption in the impulse to one or more of these parts will produce a vascular dilation through interference with the transmission of vasoconstrictor impulses from within outward; and irritation of one or more of these parts may cause a contraction of the vessels by increasing the normal stimulus sent out to local ganglia by the vasoconstrictors. An example of the general action of such irritation is shown by the increasing pressure in cerebral hemorrhage or intracranial pressure variation due to abscess or tumor, and locally, in the vascular condition met in true migraine (Starr). 1 M. Allan Starr, Jour. A. M. A., Vol. liii, No. 3, July 17, 1909, from whose article much of the material relating to vasomotor influence has been taken. Vasodilators.-The action thus far considered has been wholly of the vasoconstrictor kind, and the dilatation mentioned has been due to a cessation or reduction of the constrictor energy normally passing outward. This may be termed a passive dilatation. It is the kind produced by division of any one of the sympathetic ganglia. Experiments have shown, however, that another kind of dilatation can be produced, traceable not to a mere cessation of constrictor impulses, but to an impulse of a positive nature sent out to local ganglia and resulting in a sudden suspension of their activity. Such an impulse is really inhibitory, arresting the action of the ganglia in spite of the continued stimuli sent to them from the central nervous system. The result is a dilatation of the arteries, produced by the blood-pressure within; this may be termed an active dilatation. An important difference has been established between these two sets of impulses, namely, that while the action of the former is constant, that of the latter is intermittent. Therefore they cannot be regarded as opponents of each other. From the anatomic standpoint the vasodilators can be traced upward in a manner similar to those controlling the vasoconstrictors, and experimental research has demonstrated the exact course of these fibers in relation to the various parts of the central nervous system, it has also been demonstrated that they exist as separate nerves, sometimes running together, although they usually enter the spinal cord at different levels. Vasomotor Reflexes.-Vasomotor reflexes are found in all parts of the body, examples of which are seen in the effect of a change of temperature, or of pain, upon local or general blood-pressure, or in the color of the face and the size of the pupil. It is probable that the major vasomotor reflexes are of a chemical nature, the chemical substance often acting directly upon the lining of blood-vessels so that this subject is closely related to the two preceding, namely, the chemical and glandular activities (see page 26). Having reviewed our present knowledge of the physiologic relations of blood-pressure and vasomotor tone, we can now approach the practical clinical side of this subject more intelligently as, through this knowledge we are more readily able to comprehend the complex changes occurring in blood-pressure in disease. For practical purposes it will be found convenient to consider the maintenance of normal blood-pressure and the production of abnormal pressures, more from the clinical standpoint, while admitting them to be dependent upon these primary physiologic divisions. This is at least the most convenient because more capable of clinical demonstration and study. From a clinical standpoint the factors concerned in the maintenance of circulatory equilibrium are: First. The energy of the heart. Second.-Peripheral resistance. Third.-Arterial tonus. Fourth.-Volume of the blood. They appear to be related to the physiologic factors in the following manner: The energy of the heart, to the mechanical arrangement, and to the nervous mechanism; peripheral resistance and arterial tonus to the chemical changes, glandular secre |