The Course and Relations of Arnold's Nerve (Auricular Branch of the Vagus) in the Temporal Bone.

IN the Journal of Anatomy and Physiology (xlvii., p. 391), I published a short paper on the " Comparative Anatomy of the Middle Ear." Among other subjects in that paper, I demonstrated the existence of a large plexus of nerves on the posterior surface of the tympanic bulla in several mammals of different orders. On account of its anatomical position, I named the structure the bullar plexus. The plexus was clearly composed of branches from at least two nerves, the facial and the vagus, and probably branches from the glossopharyngeal also took part in its formation. The existence of this structure in several mammals of different orders appeared to me to indicate that it was probably represented in man, but had hitherto escaped discovery on account of the difficulties of making a satisfactory dissection of that region in the human subject. This conjecture has proved to be justified, and it is possible to demonstrate the existence of this plexus in man, though it is a much smaller and more elusive object in him than in the other mammals referred to previously. Before going on to demonstrate the plexus itself, it is important to refer briefly to the striking anatomical differences in this region between man and the anthropoid apes on the one hand, and all other mammals on the other hand. In the first place, the mastoid process exists only in man and the anthropoid apes. The tympanic bulla, on the other hand, in these animals is reduced to an insignificant little cul-de-sac lying below and behind the tendon of the stapedius muscle, and termed in the human subject the tympanic sinus. In all other mammals the state of matters is, so to speak, reversed—that is to say, the mastoid process does not exist and the bulla is a large cavity; in many mammals, indeed, it is considerably larger than the rest of the middle-ear cavity. It must further be pointed out that the course of the facial nerve in this region is external to the bulla, but internal to