Eustachius is credited with several anatomical discoveries, among them the tensor tympani muscle and the Eustachian tube, published in his chapter entitled De auditus organis. In the last respect, however, he was anticipated by Alcmaeon, about 500 BCE. Eustachius was the first to describe the chorda tympani as a nerve. Plate VIII illustrates the “Eustachian valve”, the valvula venae cavae in the right auricle. Eustachius recognized the thoracic duct in the horse and even detected some of its valves. His work on this structure was forgotten until Aselli’s description of the lacteals. This work includes first description of the adrenals. Several of the plates deal with the structure of the kidney.

Basing his work on the dissection of fetuses and newborn children, Eustachi was the first to study the teeth in any considerable detail. In his Libellus de dentibus attached to this work he provided an important description of the first and second dentitions and described the hard outer tissue and soft inner structure of the teeth. He also attempted an explanation of the problem of the sensitivity of the tooth’s hard structure. The Libellus has a separate title page dated 1563. It was reprinted with German translation, Wien, Urban & Schwarzenberg, 1951. It was translated into English by Joan H. Thomas and edited and introduced by David A. Chernin and Gerald Shlklar as as A little treatise on the teeth. The first authoritative book on dentistry (1563) (Canton, MA, 1999). Eustachi’s illustrations of the teeth were first published in his Tabulae anatomicae, edited by Giovanni Maria Lancisi (No. 391). For further information, including a discussion of the states of the Opuscula, see the entry at HistoryofInformation.com at this link.

Digital facsimile of the 1563 edition from the Internet Archive at this link.

First edition in Latin edited by Eustachi of the glossary to Hippocrates by the first century Greek grammarian Erotianus. Erotianus's work contains the earliest list of the writings of Hippocrates, including some now lost. The Greek text alone had been printed as part of Henri Estienne's Dictionarium Medicum (1564). Eustachi based his Latin translation, accompanied by many passages in the original Greek, on a Greek manuscript in the Vatican library that was independent of Estienne's edition.
To Erotianus text Eustachi added an exhaustive commentary based on the Greek text, which it cites in the original. In addition he added in an ppendix (ff. 128-152) the first edition of his original tract De multitudine, describing the symptoms of plethora (i.e., an excess of a bodily fluid, particularly blood).

Digital facsimile from Google Books at this link.

A romantic history attaches to this fine collection of plates, drawn by Eustachius himself and completed in 1552. They remained unprinted and forgotten in the Vatican Library until discovered in the early 18th century, and were then presented by Pope Clement XI to his physician, Giovanni Maria Lancisi. The latter published them in 1714 together with his own notes. These copperplates are more accurate than the work of Vesalius. Singer was of the opinion had they appeared in 1552 Eustachius would have ranked with Vesalius as one of the founders of modern anatomy. He discovered the Eustachian tube, the thoracic duct, the adrenals and the abducens nerve, and gave the first accurate description of the uterus. He also described the cochlea, the muscles of the throat and the origin of the optic nerves. Plate XVIII is a drawing of the sympathetic nervous system. Eustachius was the first to describe the ganglion chain, but made the mistake of tracing the origin of the cervical portion to the brain-stem. With respect to dentistry, Eustachi's illustrations of the teeth, related to his Libellus de dentibus (1563-64) were first published in this work.