For the legendary Christian martyr known as Eustachius or Eustace, see Saint Eustace (legend).

Bartolomeo Eustachi (b. 1500 or 1514 - d. August 27, 1574), also known by his Latin name of Eustachius, was one of the founders of the science of human anatomy.

He came from San Severino , near Salerno, Italy, and was a contemporary of Vesalius, with whom he shares the reputation of having created the science of human anatomy. He extended the knowledge of the internal ear by rediscovering and describing correctly the tube which bears his name; and if we admit that G. F. Ingrassias anticipated him in the knowledge of the third bone of the tympanal cavity , the stapes, he is still the first who described the internal and anterior muscles of the malleus, as also the stapedius , and the complicated figure of the cochlea. He is the first who studied accurately the anatomy of the teeth, and the phenomena of the first and second dentition. The work, however, which demonstrates at once the great merit and the unhappy fate of Eustachius is his Anatomical Engravings which, though completed in 1552, nine years after the impression of the work of Vesalius, the author was unable to publish. First communicated to the world in 1714 by G. M. Lancisi , afterwards in 1744 by Cajetan Petrioli , again in 1744 by B. S. Albinus , and subsequently at Bonn in 1790, the engravings show that Eustachius had dissected with the greatest care and diligence, and taken the utmost pains to give just views of the shape, size and relative position of the organs of the human body.

The first seven plates illustrate the history of the kidneys and some of the facts relating to the structure of the ear. The eighth represents the heart, the ramifications of the vena azygos, and the valve of the vena cava, named from the author. In the seven subsequent plates is given a succession of different views of the viscera of the chest and abdomen. The seventeenth contains the brain and spinal cord; and the eighteenth more accurate views of the origin, course and distribution of the nerves than had been given before. Fourteen plates are devoted to the muscles.

Eustachius did not confine his researches to the study of relative anatomy. He investigated the intimate structure of organs with assiduity and success. What was too minute for unassisted vision he inspected by means of glasses. Structure which could not be understood in the recent state, he unfolded by maceration in different fluids, or rendered more distinct by injection and exsiccation. The facts illustarated by these figures are so important that it has been said that if the author had been fortunate enough to publish them, anatomy would have attained the perfection of the 18th century two centuries earlier at least. Their seclusion for that period in the papal library has given celebrity to many names which would have been known only in the verification of the discoveries of Eustachius.