Leonardo's Contribution to NeuroscienceThe concept that the ventricles of the brain are responsible for its major functions derives from Greek antiquity. The soul resides in the head, but since the soul is incorporeal its locus is in the cavities (i.e. fluid-filled ventricles) rather than in the surrounding brain tissue. Some Greek scientists, most notably Aristotle, argued that the heart and not the head is the seat of sensation and cognitive functions; one loses consciousness upon excessive bleeding, and many emotions are experienced viscerally (e.g. one has a "gut feeling" or an emotional "heart-ache"). However the majority of philosophers and scientists assigned mental functions to the brain. By Leonardo's time, the ventricular theories had become entrenched in philosophy. The brain was thought to contain an anterior ventricle (typically thought to contain the senso comune, and also phantasy and imagination); a second ventricle that mediated cognition; and a posterior ventricle that serves memory.
Leonardo da Vinci described the imprensiva, a brain structure that mediates between sense organs (such as the eye) and the senso comune. The term imprensiva has not been adopted by any anatomist before or after Leonardo. He is quoted in Richter no. 836 (C. Atl. 89a; 258a):
The Common Sense [senso comune], is that which judges of things offered up to it by the other senses. The ancient speculators have concluded that that part of man which constitutes his judgment is caused by a central organ to which the other five senses refer everything by means of impressibility [impressiva]; and to this centre they have given the name Common Sense. And they say that this Sense is situated in the centre of the head between Sensation and Memory. And this name of Common Sense is given to it solely because it is the common judge of all the other five senses i.e. Seeing, Hearing, Touch, Taste and Smell. This Common Sense is acted upon by means of Sensation which is placed as a medium between it and the senses. Sensation is acted upon by means of the images of things presented to it by the external instruments, that is to say the senses which are the medium between external things and Sensation. In the same way the senses are acted upon by objects. Surrounding things transmit their images to the senses and the senses transfer them to the Sensation. Sensation sends them to the Common Sense, and by it they are stamped upon the memory and are there more or less retained according to the importance or force of the impression. That sense is most rapid in its function which is nearest to the sensitive medium and the eye, being the highest is chief of the others. Of this then only we will speak, and the others we will leave in order not to make our matter too long. Experience tells us that the eye apprehends ten different natures of things, that is: Light and Darkness, one being the cause of the perception of the nine others, and the other its absence:--Colour and substance, form and place, distance and nearness, motion and stillness.
Leonardo da Vinci, quoted in Richter no. 838 (W. An. II. 202a [B]):
How the five senses are the ministers of the soul
The soul seems to reside in the judgment, and the judgment would seem to be seated in that part where all the senses meet; and this is called the Common Sense and it is not all-pervading throughout the body, as many have thought. Rather is it entirely in one part. Because, if it were all-pervading and the same in every part, there would have been no need to make the instruments of the senses meet in one centre and in one single spot; on the contrary it would have sufficed that the eye should fulfil the function of its sensation on its surface only, and not transmit the image of the things seen, to the sense, by means of the optic nerves, so that the soul-for the reason given above-may perceive it in the surface of the eye. In the same way as to the sense of hearing, it would have sufficed if the voice had merely sounded in the porous cavity of the indurated portion of the temporal bone which lies within the ear, without making any farther transit from this bone to the common sense, where the voice confers with and discourses to the common judgment. The sense of smell, again, is compelled by necessity to refer itself to that same judgment. Feeling passes through the perforated cords and is conveyed to this common sense. These cords diverge with infinite ramifications into the skin which encloses the members of the body and the viscera. The perforated cords convey volition and sensation to the subordinate limbs. These cords and the nerves direct the motions of the muscles and sinews, between which they are placed; these obey, and this obedience takes effect by reducing their thickness; for in swelling, their length is reduced, and the nerves shrink which are interwoven among the particles of the limbs; being extended to the tips of the fingers, they transmit to the sense the object which they touch. The nerves with their muscles obey the tendons as soldiers obey the officers, and the tendons obey the Common Sense as the officers obey the general. Thus the joint of the bones obeys the nerve, and the nerve the muscle, and the muscle the tendon and the tendon the Common Sense. And the Common Sense is the seat of the soul, and memory is its ammunition, and the impressibility [imprensiva] is its referendary since the sense waits on the soul and not the soul on the sense. And where the sense that ministers to the soul is not at the service of the soul, all the functions of that sense are also wanting in that man's life, as is seen in those born mute and blind.