Let it be noted that the vagus nerves (right and left) of which the recurrent laryngeal are branches, originate from the base of the brain, pass down the neck beneath the jugular vein in company with the carotid artery; that on entering the chest the right vagus nerve gives off its recurrent branch which proceeds at once up the neck along the course of its parent trunk till it reaches the larynx, to the muscles on the right side of which it is distributed; that the left vagus nerve on the other hand proceeds backward in company with the left innominate artery as far as the base of the heart, where on about the level of the space between the sixth and seventh ribs it gives off the left recurrent nerve; that this left recurrent nerve closely applied at its point of origin to the great parent arteries turns round the posterior aorta enclosing it in a loop, and gaining the lower end of the windpipe follows its course to the larynx. It will thus be understood how many chest diseases may implicate the left recurrent nerve, and from which the right, which extends no deeper than between the two first ribs, may be completely exempt. The frequent supervention of roaring as a sequel of chest diseases receives in this an ample explanation. Its connection with pleurisy becomes especially probable, as the nerve lies in contact with the surface of the pleura alike in its descending and ascending course within the chest. Finally the loop encircling the posterior aorta exposes the nerve to constant stretching and shocks from the heart’s action during violent exertions and in excited states of the circulation generally. Vaerst and Sussdorf show that the nerve is habitually flattened between the posterior aorta and trachea, the effect being worst when the heart’s action is excited. It remains to notice a few instances in which dissection established the connection of interference with the nerve at some part of its course and the existence of roaring. (a.) Godine found in a roarer a tumor about the size of a chicken’s egg, pressing on the commencement of the pulmonary artery. He attributed the roaring to the impaired circulation of blood in the lungs by the pressure on the artery. Considering that the tumor must have been precisely in the situation of the left recurrent nerve at its point of origin, it becomes much more probable that the symptom resulted from pressure on this nerve. (b.) The elder Bouley found in one case a considerable engorgement of the group of lymphatic glands in the anterior part of the chest and through the centre of which the left recurrent nerve passed.