In less than fifty pages Vesalius describes the contents of the thorax. He writes intelligently of the membrane lining the thorax, and then gives an account of the arteria aspera, as the trachea was formerly named. Passing on to the lungs, he next takes up the anatomy of the heart. He describes its position, form, and structure in better terms than had been done by preceding anatomists. The auricles, ventricles, and valves are carefully examined. His illustrations of both lungs and heart are excellent. In the 1543 edition of the _Fabrica_, Vesalius adopts the erroneous view of Galen that openings exist in the septum of the heart. In the second edition of his book, published in 1555, he says that influenced by the views of Galen, he believed that the blood passes from the right to the left ventricle of the heart, through the septum, by means of the pores. Vesalius immediately adds that the septum of the heart is as dense and compact as the rest of this organ, and that not the smallest quantity of blood passes through the septum. His account of this subject is best given in his own words:—“In recounting as above the structure of the heart, and the use of its different parts, I have followed in the main the doctrines of Galen; not that I regard them in all particulars as consonant with the truth, but because, in attributing new functions and uses to a number of parts, I am still distrustful of myself, and not long ago should hardly have ventured to differ from that Prince of Physicians by so much as a finger’s breadth. As for the dividing wall, or septum, between the ventricles forming the right side of the left cavity, the student of anatomy should consider carefully that it is equally thick, compact, and dense, with all the rest of the cardiac substance enclosing the left ventricle. And accordingly, notwithstanding what I have said about the pits in this situation, and at the same time not forgetting the absorption by the portal vein from the stomach and intestines, I still do not see how even the smallest quantity of blood can be transfused, through the substance of the septum, from the right ventricle to the left”. Vesalius and other anatomists knew of the hepatic circulation, or at least believed in some communication between the portal and hepatic veins:—“The branches of this vein”—vena cava—“distributed through the body of the liver, come in contact with those of the portal vein; and the extreme ramifications of these veins inosculate with each other, and in many places appear to unite and be continuous”. Vesalius knew that in several particulars the accepted physiology of the vascular system was wrong. If he could have lived a few years longer, it is possible that he might have solved the great problem which was made clear by William Harvey. In the light of our present knowledge some of Vesalius’s words are suggestive: