LCL 288: 390-391
(The divisibility of plants and some of the lower animals, which somewhat obscures the application of this law, is due to their being agglomerations rather than biological units.) The heart or its analogue is found in the centre of the body; being the seat of the soul, it is the dominant organ, and supplies the heat which is necessary to maintain life.
There are two ways in which this heat may fail: by extinction, when it has too little fuel (i.e., food), and by exhaustion, when it has too much; or rather when the fuel is consumed too quickly. To prevent over-rapid combustion some form of refrigeration is necessary.
Refrigeration and Respiration. Among the lower forms of life combustion is retarded in various ways. Plants normally obtain enough refrigeration from the air or water which surrounds them, and the same is true of most very small and bloodless “animals.” Buzzing insects have a peculiar method: they expand and contract the air contained in their bodies, thus forcing it through their narrow waists, where it is cooled by the surrounding air; in its passage to and fro it sets up friction against a membrane (the nature and position of which is not clear) and so produces a musical note. Fishes regulate their vital heat by taking in water through their gills. Animals generally, however, obtain refrigeration through respiration, although some which have bloodless and spongy lungs (e.g., many ovipara) do not respire much; the lungs when once charged with air remain cool for a long time.
Aristotle’s account of the mechanics of respiration is close enough to the truth and calls for little comment, apart from the all-important fact that the
process is conceived and described as purely refrigerative. One might have supposed that blood is actually cooled in the lungs, but it seems clear from Hist. An. 496 a 32 and the description of the analogous process in the case of fishes (478 b 10 ff.) that the inhaled air is supposed to pass from the lungs to the heart (through the pulmonary arteries?) and so to temper the vital heat at its very centre. This leads us on to the explanation of another phenomenon.
Pulsation. Knowing nothing of the circulation of the blood or of the distinction between veins and arteries, Aristotle accounts for pulsation by an ingenious theory. We have already seen in the Introduction to On the Soul (p. 7) the function of the σνμφυτον πνεῦμα or “connatural breath” in transmitting psychical stimuli to the physical parts of an organism. Unhappily Aristotle never explains clearly what he understands by this πνεῦμα. Obviously it is not the same thing as ordinary breath. The epithet σύμφυτον is usually attached to make the distinction clear; but it is often omitted. One might have supposed that πνεῦμα is peculiar to living organisms; but in G.A. 762 a 20 we are told that “water is present in earth, and πνεῦμα in water, and soul-heat in all πνεῦμα so that in a sense all things are full of soul.” It is especially connected with semen: G.A. 736 a 1 “semen is a compound of πνεῦμα and water—πνεῦμα is hot air”; and again in 736 b 30 ff. the fertility of semen is ascribed to the presence in it of the hot substance which is not fire but “the πνεῦμα which is enclosed within the semen or foam-like stuff, and the natural substance which is in the πνεῦμα; and this substance is analogous to the element which belongs to the stars” (Peck). This element is the