Colombo was the first to propose the pulmonary circuit in an intellectual climate that could expand on his theory. In addition to the pulmonary circuit, Colombo also discovered that the main action of the heart was contraction, rather than dilation as had previously been thought. Both of these discoveries were later confirmed by William Harvey. In addition, Colombo is credited with coining the term "placenta" and in his work describes the placenta as a place where pure and perfect blood is stored for the baby before birth. He believes Galen misinterpreted the placenta when it comes to humans and challenges Galen’s views on the placenta because Galen only performed dissections on animals. Colombo argues that performing vivisections helps one to understand how the system works as a whole. At the time his work in this anatomical area was revolutionary and provided the basis for the understanding of the placenta and other anatomical structures we have today. These detailed descriptions and critiques are in his book "De Re Anatomica Libri XV". Much cited is Columbo's naming and description of the clitoris as "Amor Veneris, vel Dulcedo Appelletur." He stated, "It should be called the love or sweetness of Venus." While Colombo was not the first to discover the clitoris, he is one of the first to propose its role in female sexual pleasure. This finding caused quite a stir amongst the general public as it was already thought that females had anatomy corresponding to their male counterparts; the addition of a bodily structure could cause women to be viewed as hermaphrodites. Politically, this had implications as it was suggested that women could be anatomically equal to men. The structure of Colombo's book, "On anatomizing", is indicative of the way in which Colombo went about dissecting subjects for his research. Not only was the order and structure of this work very thought out, but also it differed from the structure of other anatomists at the time. Colombo's anatomical text consisted of 15 different books, each of which covered information concerning a different part of the body. Book 1 described the bones, while Book 2 and Book three, respectively, outlined the cartilages and ligaments that could be found within the human body. Book 4 explains the skeleton as a whole, bringing together the individually discussed anatomical elements described in the preceding three books. The muscles of the human body are described in Book 5, and the liver and veins share the content of Book 6.