In order to understand why all these authors invoked the existence of the third ventricle, regarded by some of them as a passage between the other two, we must turn to the physiological beliefs of the age. It must be recalled that before the demonstration of the circulatory movement of the blood a certain amount of communication was believed to exist between right and left ventricles. The complicated nature of the ventricular cavities and the intricacy of the columnae carneae promoted the idea of the presence of minute passages in the interventricular septum. Even so astute an observer as Leonardo da Vinci considered that ‘the ventricles are separated by a _porous_ wall, through which the blood of the right ventricle penetrates into the left ventricle, and when the right ventricle shuts, the left opens and draws in the blood which the right one gives forth’ (Plate XXXVIII_b_).[215] [Illustration: Fig. 22. From Johannes Adelphus, _Mundini de omnibus humani corporis interioribus menbris Anathomia_, Strassburg, 1513. The diagram shows the two lateral ventricles and the ‘central’ ventricle. By a printer’s error the letters _c_ and _d_ are transposed. The _arteria adorti_ is the aorta, the _arteria venalis_ the pulmonary vein, the _vena chilis_ the vena cava, and the _vena arterialis_ the pulmonary artery. The auricles are ignored, as is frequently the case in works of the period, and the pulmonary veins are represented as opening directly into the ventricles.] Although the third ventricle is described in all the twenty-five editions of Mondino, many of which are illustrated, they present no drawing of it except the wretched little diagram of J. A. Muelich (Johannes Adelphus) in 1513, which we here reproduce (Fig. 22). The confusion, however, to which the idea of a third ventricle gave rise influenced anatomy almost as late as the seventeenth century, and is illustrated in the anatomical figures of a late edition of Hans von Gersdorff (1556),[216] where the trachea is actually shown opening into the left ventricle (Fig. 23). It was Vesalius who took the first great step towards the discovery of the circulation of the blood, by firmly maintaining that the interventricular septum was solid and contained neither passages nor intermediate ventricle.[217] [Illustration: Fig. 23. From Hans von Gersdorff, _Feldt und Stattbüch bewerter Wundartznei_, Frankfurt, 1556. The trachea (_d_) is represented as opening directly into the heart.] VI. ITALIAN TEXT MS. Canonici Ital. 237 _Hyeronimi manfredi ad Magnificum & potentem dominum ac militem Iohannem Bentiuolum insequens opus de corporis humani anothomia exordium._