Cardiovascular reflexes

DURING an investigation of the "Central and Reflex Regulation of the Circulation," Anrep and Starling(l) were unable to obtain evidence of a reflex mechanism being involved in the production of cardiac slowing in response to a rise in aortic pressure'. They used the crossed circulation method described by Anrep(2) in which the head derived its blood supply from a heart-lung preparation and the rest of the animal received blood from its own heart. It occurred to us that the negative results obtained might have been due to a rise in venous pressure occurring simultaneously with the rise in aortic pressure and so neutralising any tendency for the heart rate to become slower (Bainbridge(3)). The possibility that the low carbon dioxide content of the blood perfusing the brain contributed to their results was also considered; they did, however, use a carbon dioxide-air mixture for ventilation of the heartlung preparation in some of their experiments. In order to study the problem further we devised a method in which the aortic pressure, the head arterial pressure and the head blood flow could be varied at will in the intact animal. Care was taken not to overventilate the lungs. Method. Dogs were used in all experiments and were fully anaesthetised with chloralose, 1 decigram per kilo body-weight being injected intravenously. Morphia was not administered. The blood of the animal was defibrinated and artificial respiration applied; a carbon dioxide mixture was not used. A screw clamp was placed on the thoracic aorta as low down in the chest as possible. A cannula was inserted into the central end of the subclavian artery and counected to a Pavlov and Stolnicov stromuhr (4, 5), similar to the pattern recently used by Anre-p. The brachio-cephalic artery was then tied close to its origin from the aorta and a cannula inserted pointing towards the brain; this was connected to the other end of the st-romuhr. In two experiments the