In any stage of partial block, pressure on the vagus nerve in the neck produces certain specific changes. (Fig. 52.) Robinson and Draper[13] have found qualitative differences in the two vagi. The right vagus sends most of its fibers to the s-a node (Fig. 53) and has a more evident influence on the rate and force of the cardiac contractions. The majority of fibers from the left vagus are distributed to the A-V node so that its most evident action is upon the conductivity of the impulse. Pressure then on the right vagus will have a tendency to slow the whole heart. Pressure on the left vagus will have a tendency to prolong the P-R interval until even complete block occurs. Even when the heart block is complete, stimulation of the accelerator nerve, as a rule, increases the rate of both auricles and ventricles. [Illustration: Fig. 52.--Influence of mechanical pressure on the right vagus nerve. (After Eyster and Evans.)] [Illustration: Fig. 53.--Schematic distribution of right and left vagus. (After Hart.)] [13] Jour. Exper. Med., 1911, xiv, 217. If the block is functional, depending upon some temporary overstimulation of the vagus nerve, atropin, which paralyzes the endings of the vagus, will naturally lift the block. If the block is due to some actual lesion of the bundle of His, such as fibrosis, gumma, or other lesion, then atropin will have no influence to terminate the block. In this manner we are able to distinguish between functional and organic heart block. CHAPTER V BLOOD PRESSURE IN ITS CLINICAL APPLICATIONS It is well to bear constantly in mind the point made over and over in this work, that blood pressure is only one of many methods of acquiring information. He who worships his sphygmomanometer as a thing apart and infallible will sooner or later come to grief. Judgment must be used in interpreting changes in blood pressure just as judgment is essential in properly evaluating any instrumental help in diagnosis. One must not forget the personal equation which enters into even accurate instrumental recording in medicine and surgery. In this chapter there will be no attempt to quote largely from what others have said or thought. Every one has his own opinion as to the value of certain methods after he has worked with them for a long time. The ideas here expressed, except in cases where no opportunity has offered to make personal studies, are those gathered from personal experience. =Blood Pressure in Surgery=