. An introductionto physiology. ain to the ganglion of thevagus (Fig. 55), and thence down the vagus trunkto the heart. Thus in the greater part of itscourse the vagus cannot be stimulated withoutexciting both the augmentor and the inhibitorycardiac fibres. To excite either alone it is neces-sary to stimulate the respective nerves abovetheir junction. Preparation of the Sympathetic. — Cut away thelower jaw of a large frog, the brain of which hasbeen destroyed by pithing, and continue the slitfrom the angle of the mouth downwards for ashort distance. Avoid cutting the vagus nerve(Fig. 56). Turn
. An introductionto physiology. ain to the ganglion of thevagus (Fig. 55), and thence down the vagus trunkto the heart. Thus in the greater part of itscourse the vagus cannot be stimulated withoutexciting both the augmentor and the inhibitorycardiac fibres. To excite either alone it is neces-sary to stimulate the respective nerves abovetheir junction. Preparation of the Sympathetic. — Cut away thelower jaw of a large frog, the brain of which hasbeen destroyed by pithing, and continue the slitfrom the angle of the mouth downwards for ashort distance. Avoid cutting the vagus nerve(Fig. 56). Turn the parts well aside, and exposethe vertebral column where it joins the the mucous membrane covering theroof of the mouth. The sympathetic is situated 284 THE CIRCULATION OF THE BLOOD immediately under the levator anguli scapulaemuscle, which must be carefully removed. Thenerve will then be visible. It is commonly pig-mented and usually lies under an artery. Care-fully isolate the nerve. Put a ligature around it. LA6 Fig. 65. Scheme of the sympathetic nerve in the frog. OC. Levator anguli scapulae. Sym. Sympathetic. GP. Glosso-pharyn-geus. V-S. Vago-sympathetic. G. Ganglion of the vagus. Ao. Subclavian artery. (After Stirlings reproduction of Gaskell andGadows plate.) as far away from the skull as practicable, andcut the nerve caudal to the ligature. Action of the Sympathetic on the Heait. —Arrange the inductorium for weak tetanizing cur-rents. In the primary circuit place the electro- INNERVATION OF HEART AND BLOOD-VESSELS 285 magnetic signal. Prepare the sympathetic asdirected above. Expose the heart (page 75).Place it in the heart-holder. Should the heartbeat rapidly, slow it with ice. Let the writingpoint record above the point of the electromag-netic signal on a drum revolving so slowly thatthe individual beats shall appear in the curvevery close together, yet far enough apart to bereadily counted. Divide the observation intonine pe
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