Now since the heart attracts by this orifice of the vena chilis more than it expels, therefore nature ordains that in the moment of contraction when the blood is expelled this orifice closes, and when the heart dilates it opens.
Moreover there are three little valves (hostiolitti) or doors opening from without inward, and these valves are not very depressed; so that by this same orifice only part of the purified blood is expelled to the other members, because part goes to the lungs and the remainder forms the vital spirit; therefore nature ordains that these valves do not entirely close. From the vena chilis, before it enters the cavity of the heart, there arises another vein, which surrounds the root of the heart; and from it are given off branches which disperse themselves through the substance of the heart, and from the blood of that vein the heart nourishes itself.
The right ventricle towards the lung has another orifice into which opens the arterial vein, bringing the blood from the heart to nourish the lung: in this orifice also are three valves (hostioli) opening from within outward and closing from without inward, in the opposite way to the valves of the other orifice; and this is so that they should entirely close. Hence by this orifice the heart during the period of contraction can expel, and yet during the period of its dilatation cannot attract anything through it as was done in the first orifice.
The left ventricle of the heart has its sides denser and thicker than the sides or walls of the right ventricle; and this for three reasons: Firstly, because in the right ventricle is contained the blood, which is heavy, while in the left ventricle there is spirit, which is very light; therefore in order that the heart should not be heavier and more ponderous on one side than on the other, it was necessary to compensate in this manner, that is, that the left ventricle should be thicker in its walls than the right. In the second place, the spirit being more subtil and more volatile (resolubile) than blood, it needs a stronger habitation and better supports. Thirdly, the left ventricle is much warmer than the right, because in it is generated the spirit from the blood, by a great heat which makes that blood more subtil; and heat is better preserved in a substance that is dense and thick.
In the cavity of this ventricle near its root are two orifices: one is the orifice of an artery called artharia adorti [= aorta], because it has immediate origin in the heart and because it is the source of all the others: by this artery the heart sends the generated spirit to all the members; and the very subtil blood is mixed with the spirit when the heart contracts. For which reason there are at the entrance of this orifice three valves, which close entirely from the outside inwards; and they open from the inside outwards, and this orifice is very deep.
The other orifice is that of the venal artery which conveys the air from the lung to cool the heart and transports warm vapours from the heart to the lung as has been said above; and in this orifice are two valves which do not entirely close: and they are well raised so that they can better apply themselves to the sides [edges] of the heart when it sends out the spirit: these are marvellous works of nature, as is also the central ventricle of the heart, for this ventricle has not one cavity but many; these are small but wide, and more numerous on the right than on the left; and nature contrived thus, so that the blood which goes from the right ventricle to the left to be converted continually into spirit becomes thin in these cavities.
And by this thou canst see that four things have birth in the heart. The first is the artery called adorti, the second is the vena chilis, the third is the arterial vein, and the fourth the venal artery.
Also thou wilt see in the heart certain membranous parts like auricles, or rather like small ears, able to dilate or contract: these are contrived by nature in order that when overmuch blood or spirit is generated the heart can dilate so as to contain it; and also that the heart may contract when there is no such abundance.
And it is here that Galen asks, Why did not nature make the heart so large that it could contain every increase of blood or spirit without the addition of these membranes? Galen replies that this was first because the heart would have been too large and therefore too heavy; secondly, because as it is not always generating a great quantity of blood and spirit, if the heart had been too large, its cavity would usually have been empty: but these auricles dilate with the accumulation of blood or spirit, and contract with its decrease.