Observationes anatomicae, a work of 232 leaves printed in the comparatively small octavo format, with no illustrations, was the only work Fallopio published before his death from tuberculosis at age thirty-nine, and is thus the only one that can be said to be fully authentic. The remainder of Falloppio's works were edited for publication from his lecture notes, and may represent more or less than the author's original intention. Observationes was not an all-inclusive textbook of anatomy but rather a detailed critical commentary on Vesalius's De humani corporis fabrica (1543), in which Falloppio attempted to correct errors in the earlier work, and to add material that Vesalius had overlooked; for this reason, there was no need for illustrations. The large amount of new material included Falloppio's investigations of primary and secondary centers of ossification, the first clear description of primary dentition, numerous contributions to the study of the muscles (especially those of the head), and the famous account of the uterine ("Falloppian") tubes, which he correctly described as resembling small trumpets (tubae), definitely proved the existence of the seminal vesicles. He also gave to the placenta and vagina their present scientific names, provided a superior description of the auditory apparatus (including the first clear accounts of the chorda tympani and semicircular canals), and was the first to clearly distinguish the trochlear nerve of the eye. Vesalius responded positively to Falloppio's work with his posthumously published Examen on Falloppio (1564).

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