If any one will perform Galen’s experiment of dividing the trachea of a living dog, forcibly distending the lungs with a pair of bellows, and then tying the trachea securely, he will find, when he has laid open the thorax, abundance of air in the lungs, even to their extreme investing tunic, but none in either the pulmonary veins or the left ventricle of the heart.  But did the heart either attract air from the lungs, or did the lungs transmit any air to the heart, in the living dog, much more ought this to be the case in the experiment just referred to.  Who, indeed, doubts that, did he inflate the lungs of a subject in the dissecting—­room, he would instantly see the air making its way by this route, were there actually any such passage for it?  But this office of the pulmonary veins, namely, the ransference of air from the lungs of the heart, is held of such importance, that Hieronymus Fabricius of Aquapendente, contends that the lungs were made for the sake of this vessel, and that it constitutes the principal element in their structure.  But I should like to be informed why, if the pulmonary vein were destined for the conveyance of air, it has the structure of a blood—­vessel here.  Nature had rather need of annular tubes, such as those of the bronchi in order that they might always remain open, and not be liable to collapse; and that they might continue entirely free from blood, lest the liquid should interfere with the passage of the air, as it so obviously does when the lungs labour from being either greatly oppressed or loaded in a less degree with phlegm, as they are when the breathing is performed with a sibilous or rattling noise.

Still less is that opinion to be tolerated which, as a two-fold material, one aerial, one sanguineous, is required for the composition of vital spirits, supposes the blood to ooze through the septum of the heart from the right to the left ventricle by certain hidden porosities, and the air to be attracted from the lungs through the great vessel, the pulmonary vein; and which, consequently, will have it, that there are numerous porosities in the septum of the heart adapted for the transmission of the blood.  But by Hercules! no such pores can be demonstrated, nor in fact do any such exist.  For the septum of the heart is of a denser and more compact structure than any portion of the body, except the bones and sinews.  But even supposing that there were foramina or pores in this situation, how could one of the ventricles extract anything from the other—­the left, e.g., obtain blood from the right, when we see that both ventricles contract and dilate simultaneously?  Why should we not rather believe that the right took spirits from the left, than that the left obtained blood from the right ventricle through these foramina?  But it is certainly mysterious and incongruous that blood should be supposed to be most commodiously drawn through a set of obscure or invisible ducts, and air through perfectly open passages, at one and the same