The Carotid Sinus Mechanism

THE carotid sinus is a small dilatation present on the commencement of the internal carotid artery. It has so little obvious importance that, until recently, it was neglected by anatomists and physiologists as an uninteresting detail of blood-vessel anatomy. Adachil states that it is present in eighty-five per cent. of cases, but I have found the dilatation constantly present in a series of fifty-two adults (104 carotid arteries). Closely associated with this swelling is the carotid body which has been thought to be an abberant portion of chromaffin tissue. The researches of Hering,2 de Castro,3 Heymans,4 and others have shown that these two structures, far from being mere incidentals of body structure, are of very considerable importance in controlling the circulation and, possibly, the chemical balance of the blood. Parry in 1799 described slowing of the pulse following digital pressure on the carotid vessels. Tschermak5 rediscovered this phenomenon in 1868, but he attributed the result to pressure on the vagus nerve with consequent stimulation of the vagal cardio-inhibitory fibres. This explanation was accepted by most physiologists until Hering published his work. Hering showed definitely that the bradycardia and the fall in blood-pressure following digital pressure on the side of the neck in the neighbourhood of the carotid bifurcation was due to compression, not of the vagus nerve, but of the carotid sinus. He suggested, on physiological grounds, that the nerve-endings of the carotid branch of the glosso-pharyngeal nerve were stimulated by such pressure, and he expressed the opinion that the nerve endings in the artery wall respond to the stimulus of increased internal pressure within the artery and subsequent expansion of the artery wall.* Hering's suggestion was demonstrated to be correct by de Castro, who showed that the nerve-endings of the carotid branch of the glosso-pharyngeal nerve (now called the sinus nerve) are the receptor-mechanism of such stimuli. He also showed that the histology of the sinus nerve-endings was very similar to that of those found in the wall of the arch of the aorta and associated with the cardiac depressor nerve. This comparison is of particular interest when it is remembered that in embryonic life the commencement of the internal carotid artery is the third branchial arch artery, the aorta is the fourth branchial arch artery, and the nerves going to these branchial arches are the glosso-pharyngeal and a branch of the vagus. There is, thus, anatomical reason to homologize the aorta with the carotid sinus, and this homology fits well with the known analogy of their depressor mechanisms.