Book IV provides an account of the nervous system. It is introduced by an attempt to clarify and limit the meaning of the word “nerve” to the vehicle transmitting sensation and motion, because “leading anatomists declare that there are three kinds of nerve” : ligament, tendon, and aponeurosis. “From dissection of the body it is clear that no nerve arises from the heart as it seemed to Aristotle in particular and to no few others.” Although Vesalius was obliged to accept the Galenic explanation of nervous action as induced by animal spirit distributed through the nerves from the brain, his examination of the optic nerve led him to the conclusion that the nerves were not hollow, as Galen had asserted. “I inspected the nerves carefully, treating them with warm water, but I was unable to discover a passage of that sort in the whole course of the nerve.”
Vesalius accepted Galen’s classification of the cranial nerves into seven pairs even though he recognized more than that number and described a portion of the trochlear nerve. To avoid confusion he declared that he would “not depart from the enumeration of the cranial nerves that was established by the ancients.” Although he was not wholly successful in his efforts to trace the cranial nerves to their origins, and despite some confusion about their peripheral distribution, the level of knowledge in the text and illustrations was well above that of contemporary works and was not to be surpassed for about a generation. Vesalius was more successful in tracing the spinal nerves, but on the whole the account of the nerves must be described as being of lesser quality than some of the other books.
The description of the abdominal organs in book V is detailed and reasonably accurate. Since he knew of no alternative Vesalius accepted that aspect of Galen’s physiology which placed the manufacture of the blood in the liver. Nevertheless he denied not only that the vena cava takes its origin from the liver but also that the liver is composed of concreted blood. Here his strongest blow against Galen and medieval Galenic tradition was his denial, based on human and comparative anatomy, of the current belief in the liver’s multiple (usually five) lobes. According to Vesalius the number of lobes increased with the descent in the chain of animal life. In man the liver had a single mass, while the livers of monkeys, dogs, sheep, and other animals had multiple lobes that became more numerous and more clearly apparent. This difference once again proved the error of dependence upon nonhuman materials.
Vesalius also denied the erroneous Galenic belief that there was a bile duct opening into the stomach as well as one into the duodenum. In regard to the position of the kidneys, he had begun to move away from the erroneous view expressed in the Tabulae anatomicae that the right kidney was placed higher than the left. Although this error is illustrated in the Fabrica, the text declares that the reverse could also be true. Despite this partial error of traditionalism. Vesalius denied a second traditional opinion that the urine passed through the kidneys by means of a filter device. The filter theory had also been denied by Berengario da Carpi; but Vesalius went a step further by asserting that the “serous blood” was deliberately selected or drawn into the kidney’s membranous body and its “branchings” to be freed of its “serous humor” in the same way that the vena cava was able to select and acquire blood from the portal vein, and that the excrement was then carried by the ureters to the bladder.
The book ends with a discussion of human generation and the organs of reproduction. Although Vesalius denied the medieval doctrine of the seven-celled uterus and declared the traditional representation of the horned uterus to result from the use of animal specimens, his description of the fetus and fetal apparatus was of less significance, reflecting, as he admitted, the lack of sufficient pregnant human specimens.
Book VI describes the organs of the thorax. It is chiefly important for the description of the heart, which Vesalius described as approaching the nature of muscle in appearance, although it could not be true muscle since muscle supplied voluntary motion and the motion of the heart was involuntary. In this instance Vesalian principle bowed to Galenic theory, and recognition of the muscular substance of the heart had to await William Harvey’s investigations in the next century.
Like all his contemporaries Vesalius regarded the heart as formed of two chambers or ventricles. The right atrium was not considered to be a chamber but rather a continuation of the inferior and superior venae cavae, considered as a single, extended vessel; and the left atrium was thought to be part of the pulmonary vein. According to Galen the ventricles were divided by a midwall containing minute openings or pores through which the blood passed or seeped from the right ventricle into the left, an opinion that Vesalius strongly questioned even though by implication he was casting doubt on Galen’s cardiovascular physiology. “The septum of the ventricles having been formed, as I said, of the very thick substance of the heart… none of its pits-at least insofar as can be ascertained by the senses-penetrates from the right ventricle into the left. Thus we are compelled to astonishment at the industry of the Creator who causes the blood to sweat through from the right ventricle into the left through passages which escape our sight.” Finally Vesalius gave strong expression to his opinion of ecclesiastical censorship over the question of the heart as the site of the soul. After referring to the opinions of the major ancient philosophers on the location of the soul, he continued:
Lest I come into collision here with some scandalmonger or censor of heresy, I shall wholly abstain from consideration of the divisions of the soul and their locations, since today… you will find a great many censors of our very holy and true religion. If they hear someone murmur something about the opinions of Plato, Aristotle or his interpreters, or of Galen regarding the soul, even in anatomy where these matters especially ought to be examined, they immediately judge him to be suspect in his faith and somewhat doubtful about the soul’s immortality. They do not understand that this is a necessity for physicians if they desire to engage properly in their art…
The seventh and final book provides a description of the anatomy of the brain, accompanied by a series of detailed illustrations revealing the successive steps in its dissection. Until the time of Vesalius, illustrations of the brain and any accompanying text usually stressed the localization of intellectual activities in the ventricles, with perception in the anterior ventricles, judgment in the middle, and memory in the posterior. Sensation and motion were considered the work of animal spirit produced in a fine network of arteries at the base of the brain, the rete mirabile. The existence of the rete mirabile in the human brain had been questioned by Berengario da Carpi. It was now firmly denied by Vesalius, who showed the belief in this organ to have been the result of dissection of animals, since such an arterial network does in fact exist in ungulates. Vesalius was also the first to state that the ventricles had no function except the collection of fluid. Moreover, he denied that the mind could be split up into the separate mental faculties hitherto attributed to it. As a corollary he intimated that although animal spirit affected sensation and motion, it had nothing at all to do with mental activity-in short he suggested a divorce between the physical and mental animal. The discussion of the brain is concluded by a chapter on the procedure to be followed for its dissection and by a final, separate section on experiments in vivisection, derived and developed mostly from experiments described by Galen. The separate treatment of this latter material indicated a recognition of physiology as a discipline distinct from anatomy.
In the Fabrica Vesalius made many contributions to the body of anatomical knowledge, by description of structures hitherto unknown, by detailed descriptions of structures known only in the most elementary terms, and by the correction of erroneous descriptions. Despite his many errors his contribution was far greater than that of any previous author, and for a considerable time all anatomists, even those unsympathetic to him, were compelled to refer to the Fabrica. Its success and influence can be measured by the shrillness of Galenic apologists, by the plentiful but unacknowledged borrowings of many, and by the avowed indebtedness of the generous few, such as Falloppio. Although Colombo, Falloppio, and Eustachi corrected a number of Vesalius’ errors and in some respects advanced beyond him in their anatomical knowledge, Colombo published his anatomical studies sixteen years after the appearance of the Fabrica, Falloppio eighteen, and Eustachi twenty. Furthermore, they relied heavily upon Vesalius’ work, the detailed nature of which made it relatively easy for others to correct or to make further contributions. Although their accomplishments deserve recognition they were built upon Vesalian foundations.
More important than the anatomical information contained in the Fabrica was the scientific principle enunciated therein. This was beyond criticism, fundamental to anatomical research, and has remained so. It was not difficult to demonstrate Galen’s errors of anatomy, but such a demonstration was only a means to an end. Its significance lay in the reason for those errors: Galen’s attempt to project the anatomy of animals upon the human body. From time to time others had pointed to Galenic errors, but no one had proposed a consistent policy of doubting the authority of Galen or of any other recognized authority until the only true source of