A depressor reaction obtainable by traction on the carotid artery

Abstract

By pulling on the carotid artery, a very marked fall of blood-pressure (often of 50 to 90 mm. Hg) and increase of respiration can be produced. Our attention was directed mainly to the fall of pressure, but most of the remarks below apply also to the respiratory phenomenon. If the carotid is divided, the phenomena can only be elicited by traction of the cephalic, but not of the cardiac end. It is therefore not due to kinkage of the aorta. It occurs after both carotids are tied, so that it is not due to cerebral anemia. It does not occur on traction of either end of the cervical vagus, depressor, or sympathetic, so that it is not produced by accidental stimulation of these nerves. It does occur after all of these nerves are divided on both sides (in dogs, cats and rabbits) so that these nerves are not concerned in the phenomena.

The following remarks apply to dogs in which both vagi were divided. Oncometric observations show that the volume of the abdominal organs decreases with the fall of blood-pressure, so that this fall cannot be vascular. In further support of this conclusion, it was found that the fall occurs after section of both splanchnics, and after clamping the aorta at the diaphragm. The fall must therefore be cardiac. The heart rate is commonly somewhat slowed, but this is neither constant nor pronounced. Myocardio-graphic tracings show that the fall of pressure is indeed due to weakening of the cardiac contractions. We have tried to trace out the path which is taken by this highly peculiar and novel reflex. By following the carotid upward, we found that the reaction could be obtained from the region of the common carotid containing the origin of the internal carotid, but from none of the other branches, or at least only to a very slight extent.