During the course of a systematic study on the human vertebrobasilar arterial system (De Caro et al. 1995), our attention was drawn in one case to a strange morphology of the ventrolateral aspect of one side of the medulla. While initially our impression was that we were facing an actual malformation, review of the structure of the brainstem as described in most standard anatomical texts showed that it could correspond to the circumolivary fasciculus (Williams et al. 1995).

The subject was a 63-y-old man who had died from rupture of an atherosclerotic aneurysm of the abdominal aorta. The medical history was negative for neuropsychiatric pathology. Brain examination showed severe atherosclerosis of the cerebral arteries and hypoplasia of the initial segment of the right posterior cerebral artery. A recent infarct was found in the white matter of the right occipital lobe. After removal of the vertebrobasilar arterial system, the right pyramid showed a superficial groove extending from about 2 mm lateral to the foramen caecum obliquely downwards to the right anterolateral sulcus of the medulla, at the level of the superior margin of the inferior root of the hypoglossal nerve (Fig. 1a). Due to this groove, the lateral part of the pyramid formed a ridge that crossed the anterolateral sulcus of the medulla passing between the 2 main roots of the hypoglossal nerve and then continued, below the inferior pole of the olive, over the lateral region of the medulla ending at the level of the posterolateral sulcus (Fig. 1b). The general appearance was that of a small cord forming a loop around the anterior margin and the inferior pole of the olive, with a descending part (8 mm long), a horizontal part (6 mm long) and with a transverse diameter of 3 mm. After formalin fixation, the brainstem was cut in 8 transverse sections. At the level of the medulla the right pyramid appeared larger than the contralateral and the small lateral cord was separated from the main part of the pyramid by a thin whitish transverse stria (Fig. 1c). Macroscopic examination of the cerebellum showed nothing of relevance.