Returning to his dissections, Leonardo now proceeded to explore the heart and system of veins in the human body. His drawings of the heart are nearly perfect. Indeed, he was probably the first to discover the endocardium membrane that sheathes the valves and sinews of the heart. Also, he pictured and described the moderator band, “the first cause of the motion of the heart.” His work on this organ led him to the doorstep of discovering the circulation of the blood—later to be carried out by William Harvey in the seventeenth century.
Further, Leonardo was the first to accurately draw a representation of the _foetus_, or unborn child, in the womb of its mother, writing in his notes that, “we conclude therefore, that a single soul governs the bodies and nourishes the two.” In addition, he drew a remarkable picture of the female figure and for the first time accurately placed her organic structure. In his notes, he also pointed the way to the laws governing metabolism when he wrote, “The body of anything whatsoever that receives nourishment continually dies and is continually renewed....” By pouring wax into a hole in the skull he made the first casts of the ventricles of the brain. Several hundred years were to pass before this method was rediscovered.
As Leonardo’s work progressed, his admiration for the complexity of the human body grew. Many times in the middle of explaining a section of anatomy he inserted a sentence or two of wonder or praise at the magnificent creation that is the human being. Indeed, these drawings and notes represent the sum of many, many dissections; moreover, Leonardo had to work under conditions that placed many obstacles in his path—the crude lights and instruments, the difficulties of obtaining corpses and, above all, the opposition of the superstitious and ignorant.
The following year Leonardo entered in his notes, “This winter of the year 1510 I look to finish all this anatomy.” And yet, however sincerely he might express such a wish, Leonardo was a person who was literally never “finished.” The scientific and artistic tasks he had chosen for himself were clearly beyond the limits of any one man. Besides, the pressures of the outside world were once more threatening the peace and quiet of his home and work.