Let's look briefly at Galen's physiological system. It involves a series of activities focused on the liver, heart/lungs, and brain, a process which must be constantly renewed if the individual is to continue living. Here is a schematic of that process, with an explanation below it:
Schematic of Galen's system of the blood
Note the following aspects, which involve the Greek concept of "pneuma", or vivifying element, present in food and the air.
1. The chyle, or digested food, is brought to the liver, where it is worked up into an impure blood, imbued with the first form of pneuma innate to all things, the natural spirits. This concoction passes into the veins, which are believed to leave from the liver.
2. This blood, charged with natural pneuma, then goes to the right chamber of the heart, where impurities are exhaled through the lungs.
3. The purified part then trickles through the invisible pores (in the pits) of the inter-ventricular septum to the left ventricle, entering it drop by drop. (These invisible pores do not exist; performation of the septum leads to "blue babies", who must be operated to repair the holes immediately upon birth if they are to live)
4. There, the blood is imbued with more pneuma, drawn from the outside by inhalation through the lungs. The net result is that the blood is now charged with a higher form of pneuma, the vital spirits.
5. This blood, along with its associated natural spirits, goes via the arteries issuing from the heart to the brain, in particular, the fine net of arteries at the base of the brain, the reta mirabile. There the blood is further refined and charged with the final and highest form of pneuma, the animal or psychic spirits.
6. The psychic spirits pass through the solid part of the brain and the ventricles of the brain and then to the nerves, which are hollow tubes. It is through the agency of the animal spirits that movement and thought are affected.
Note the following aspects of this theory:
1. Each organ has a role to play, the liver, heart and brain in particular: the liver in transforming the chyle into impure blood and imbuing it with the first form of pneuma, the natural pneuma; the heart, in further purifying the blood and charging it with the second form of pneuma, the vital spirits, and the brain which works up the highest form of pneuma, the animal spirits.
2. We have a much more concrete system of tripartite division of the soul or spirit than in Plato, for Plato limits himself to a vague referal to parts of the body (abdomen, chest, head), whereas Galen has a physiological, not a metaphysical division.
3. Anatomically, we have to accept the following claims that are now known to be false: (a) the veins issue from the liver, the arteries from the heart; (b) the septum dividing the chambers of the heart has invisible holes through which purified blood seeps from the left to the right chamber; (c) there is a fine network of arteries at the base of the brain (rete miribile) in which the animal spirits are worked up.
Nonetheless, the stature of Galen is so great that this is accepted for almost the next millenium and a half. It is against this authority that Harvey addresses himself.