Modern readers view ancient theories of blood flow through the lens of circulation. Since the nineteenth century, scholarly work on the ancient understanding of the vascular system has run the gamut from attempting to prove that an ancient author had in fact, to some extent or another, pre-empted Harvey's discovery of the circulation of the bloodFootnote 1 or towards attempting, often with some empathetic embarrassment, to explain the failure on the part of an ancient author to notice something that seems so obvious to the modern eye.Footnote 2 Thus C.R.S. Harris's 1973 book The Heart and Vascular System in Ancient Greek Medicine, which remains the standard on the topic, opens with a sentence in which he marvels at how the otherwise admirable ancient Greek physicians could have ‘failed entirely to arrive at any conception of the circulation of the blood’.Footnote 3 This modern vantage point has had an unfortunate effect. In the case of Aristotle in particular, understanding of his cardiovascular system has been diminished by a tendency to define it in contradistinction to our own modern understanding of circulation. By deliberately uncoupling from the framework of modern physiology, this paper will offer a richer and more accurate picture of his views.
To be fair, Aristotle did not help matters much. In Parts of Animals 3.4, he says definitively (Passage 1) (Part. an. 666a6–8):
ἐκ τῆς καρδίας γὰρ ἐποχετεύεται καὶ εἰς τὰς φλέβας, εἰς δὲ τὴν καρδίαν οὐκ ἄλλοθεν⋅ αὕτη γάρ ἐστιν ἀρχὴ ἢ πηγὴ τοῦ αἵματος καὶ ὑποδοχὴ πρώτη.
[Blood] is conducted out of the heart and into the veins, not from elsewhere into the heart; for this is the source or spring of the blood and its first receptacle.
This statement positively begs to be compared to circulation. It lends heavy weight to statements such as Michael Boylan's that ‘there is no return of blood . . . Aristotle's circulatory system, apart from the input from the digestive system, is a one-way system: blood moves from the heart to the uniform and non-uniform parts that require nourishment’.Footnote 4 Yet, a survey of the Aristotelian corpus indicates that this is not, in fact, an accurate description of his theories. Instead, the broader context indicates that the statement in Passage 1 above refers to the origins of the blood rather than to its subsequent movements through the heart and vascular system, which, though decidedly not circulatory, are at the very least circuitous: blood, in Aristotle's model, flows both into and out of the heart. Thus reading Passage 1 without an implicit comparison to circulation reveals that the generally agreed upon characterization of his model—that it entails the unidirectional flow of blood out of the heart and its subsequent and immediate absorption into the body—is unfounded and inaccurate. A new characterization is required.
VASCULAR SYSTEM
To understand the flow of blood in the Aristotelian vascular system, we must first understand the arrangement of that system and the nature and origin of blood. To begin with the system, Aristotle is clear that the heart is the source and centre of the blood vessels.Footnote 5 His descriptions of the heart itself, however, are contradictory, both among themselves and in relation to observable fact. Attempts to reconcile Aristotle's descriptions and map them onto the anatomy of the actual organ began in antiquity and continue through modern scholarship: an irresistible yet bedevilled project.Footnote 6 Fortunately, it is possible to address the matter at hand without first having to resolve the thorny question of his anatomical accuracy; more important for the discussion of blood flow is the far less controversial question of how he connects the chambers of the heart to each other and to the rest of the body.
Aristotle gives four distinct descriptions of cardiac anatomy, two in History of Animals, one in Parts of Animals and one in On Sleep.Footnote 7 I begin with the two passages from History of Animals, both vexed by textual difficulties. In the first, he tells us (Passage 2) (Hist. an. 496a4, 20–7):
ἡ δὲ καρδία ἔχει μὲν τρεῖς κοιλίας . . . μεγίστην μὲν τὴν ἐν τοῖς δεξιοῖς, ἐλαχίστην δὲ τὴν ἐν τοῖς ἀριστεροῖς, μέσην δὲ μεγέθει τὴν ἀνὰ μέσον⋅ ἔχει δὲ τὰς δύο μικράς⋅ καὶ εἰς τὸν πλεύμονα τετρημένας ἁπάσας, κατάδηλον δὲ κατὰ μίαν τῶν κοιλιῶν. κάτωθεν δ’ ἐκ τῆς προσφύσεως κατὰ μὲν τὴν μεγίστην κοιλίαν ἐξήρτηται τῇ μεγάλῃ φλεβί, πρὸς ἣν καὶ τὸ μεσεντέριόν ἐστι, κατὰ δὲ τὴν μέσην τῇ ἀορτῇ.
The heart has three chambers . . . the largest one on the right side, the smallest on the left, and the middle-sized one in the middle. It has two small [chambers] and all of them are perforated through to the lung and this is clear in respect to one of the chambers. And out from below the point of attachment [to the lungs], from the largest chamber it is attached to the great vein, to which the mesentery too [attaches], and from the middle [chamber] to the aorta.Footnote 8
In the next iteration, he reaffirms the three-cavity structure of the heart, though adding that in smaller animals only one or two are visible. He then adds more details (Passage 3) (Hist. an. 513a32–b7):
ἡ μεγίστη μὲν κοιλία ἐν τοῖς δεξιοῖς καὶ ἀνωτάτω αὐτῆς, ἡ δ’ ἐλαχίστη ἐν τοῖς ἀριστεροῖς, ἡ δὲ μέση μεγέθει τούτων ἐν τῷ μέσῳ ἀμφοῖν⋅ ἀμφότεραι δὲ αἱ δύο πολλῷ ἐλάττους εἰσὶ τῆς μεγίστης. συντέτρηνται μέντοι πᾶσαι αὗται πρὸς τὸν πλεύμονα, ἀλλ’ ἄδηλοι διὰ σμικρότητα τῶν πόρων πλὴν μιᾶς. ἡ μὲν οὖν μεγάλη φλὲψ ἐκ τῆς μεγίστης ἤρτηται κοιλίας τῆς ἄνω καὶ ἐν τοῖς δεξιοῖς, εἶτα διὰ τοῦ κοίλου τοῦ μέσου γίνεται πάλιν φλέψ, ὡς οὔσης τῆς κοιλίας μορίου τῆς φλεβὸς ἐν ᾧ λιμνάζει τὸ αἷμα. ἡ δ’ ἀορτὴ ἀπὸ τῆς μέσης⋅ πλὴν οὐχ οὕτως ἀλλὰ κατὰ στενοτέραν σύριγγα πολλῷ κοινωνεῖ. καὶ ἡ μὲν φλὲψ διὰ τῆς καρδίας, ἡ δ’ ἀορτὴ ἀπὸ τῆς καρδίας τείνει.
The biggest chamber is on the right side and in the highest part of the heart, the smallest is on the left side, and the middle-sized of the chambers is in between the pair of them. And these latter two are much smaller than the biggest. However, they are all perforated together into the lung, but not obviously so, except for one, because of the smallness of the passages. The great vein is hung upon the largest chamber (the one above and to the right), then [having passed] through the intermediary cavity it becomes again a vein, as if the chamber were a part of the vein in which the blood pools. The aorta [comes] from the middle chamber, except not in the same way, rather it joins on a much narrower pipe. And the vein stretches through the heart, but the aorta from it.Footnote 9
The situation changes when we turn to Parts of Animals. Here Aristotle asserts that the actual rather than apparent number of chambers in the heart varies from one to three depending on the size of the animal, with three being the ideal number. Three is ideal, because it allows for optimal functional arrangement: one chamber is required at minimum to contain the blood; two chambers is better so that each of the two main vessels, which are different in nature, can have its own, different, source; three is best so that these two sources can have one common source, which is the middle and ‘odd’ one.Footnote 10 He adds (Passage 4) (Part. an. 666b35–667a6):
τούτων δὲ πλεῖστον μὲν αἷμα καὶ θερμότατον ἔχουσιν αἱ δεξιαί (διὸ καὶ τῶν μερῶν θερμότερα τὰ δεξιά), ἐλάχιστον δὲ καὶ ψυχρότερον αἱ ἀριστεραί, μέσον δ’ αἱ μέσαι τῷ πλήθει καὶ θερμότητι, καθαρώτατον δέ⋅ δεῖ γὰρ τὴν ἀρχὴν ὅτι μάλιστ’ ἠρεμεῖν, τοιαύτη δ’ ἂν εἴη καθαροῦ τοῦ αἵματος ὄντος, τῷ πλήθει δὲ καὶ θερμότητι μέσου.
Of these [chambers], the right ones have the most and hottest blood (wherefore the right parts of the body are also hotter), the left have the least and the coldest, and the middle ones have blood that is medium in respect to quantity and heat, and it is purest. For the source ought to be as calm as possible and it can be this way when the blood is pure and intermediate as to quantity and heat.
Though he does not explicitly say so here, it would seem that he has switched the connection of the aorta to the left chamber rather than to the middle, as in History of Animals. He appears to describe a central chamber connected to the other two, which are then connected to their respective vessels.
Finally, the fourth description of the heart in On Sleep makes the suggestion in Parts of Animals explicit (Passage 5) (Somn. 458a15–19):
παντὸς δὲ τοῦ αἵματος ἀρχή, ὥσπερ εἴρηται καὶ ἐνταῦθα καὶ ἐν ἄλλοις, ἡ καρδία. τῶν δ᾿ ἐν τῇ καρδίᾳ ἑκατέρας τῆς θαλάμης κοινὴ ἡ μέση· ἐκείνων δ᾿ ἑκατέρα δέχεται ἐξ ἑκατέρας τῆς φλεβός, τῆς τε μεγάλης καλουμένης καὶ τῆς ἀορτῆς· ἐν δὲ τῇ μέσῃ γίνεται ἡ διάκρισις.
The source of all blood, as has been said both here and in other treatises, is the heart. Of the chambers in the heart, the middle is common to the others. Each of the others receives [blood] from each of the vessels, the one called great and the aorta; in the middle chamber, the separation occurs.Footnote 11
Though the two descriptions in History of Animals are more tantalizingly close to being reconcilable with anatomical fact, the descriptions in Parts of Animals and On Sleep adhere better to Aristotle's theoretical biological framework, providing a single and central source for the blood and thus also, potentially, for the soul. As a result of this, and also because the texts that contain them are more concerned with what we would today call physiology than History of Animals is, this latter model will dominate in the discussion of blood flow patterns, leaving us with a working model somewhat along the lines delineated in Figure 1.Footnote 12
Figure 1: Working model of the heart in Aristotle
The next task is to follow the aorta and the great vein (today's uena caua) as they branch out and extend to the rest of the body, creating the entire vascular system. The great vein and the aorta are the only two blood-admitting vessels that connect directly to the heart.Footnote 13 Each of them extends both above and below it, the great vein, as we saw in the second description of the heart, incorporating one of the chambers in its path, the aorta seemingly connected by an offshoot. To quote Parts of Animals, ‘the remaining vessels are outgrowths of these’.Footnote 14 By synthesizing the various scattershot descriptions of the organs and the vascular system in History of Animals, Parts of Animals and Generation of Animals, I arrive at the schema laid out in Figure 2.Footnote 15
Figure 2: Working model of the vascular system in Aristotle
With the exception of the omentum, the bladder and, depending which text you consult, the liver, spleen and uterus, each of the organs is connected to both of the main vessels.Footnote 16 Further, though the arms, for reasons connected to established medical practices, are accessible from both above and below the heart, for all the rest of the body the only path from the organs below the heart to the organs above it requires passing through the heart itself. Having established the infrastructure, so to speak, let us proceed to the contents.
BLOOD
The purpose of the heart and vascular system, Aristotle repeatedly states, is to contain the blood. Blood is essential to the life of the animal because it is the final form of nutriment; in other words, it represents the last step in assimilating ingested food into a substance that can be used to create, grow or maintain any part of the body. Thus blood is the end-product of the digestive system (of which the heart is the final organ).Footnote 17 Its final fate will be either to become flesh or to be further concocted into a residue like fat, milk or semen. Aristotle is quite explicit that blood is essentially just food that has been altered by concoction. He indicates at various points that the quality and quantity of the blood and the parts made from it are directly proportional to the quality and quantity of ingested food.Footnote 18 Thus it is clear that nutritive material must enter the heart in order for the blood to be created and in turn to come out. And, in light of the vascular arrangement just sketched, the influx and the efflux necessarily occur along the same channels.
Unfortunately, Aristotle's work On Nutrition, to which he refers repeatedly, does not survive.Footnote 19 It is nevertheless possible to cobble together an outline of his views on digestion, which is to say blood formation, from remarks in other texts. In an extended simile to plant roots in Parts of Animals, he lays out the connection of the digestive tract to the vascular system (Passage 6) (Part. an. 650a14–31):
ὥσπερ δὲ καὶ τὸ στόμα τῆς ἀκατεργάστου τροφῆς πόρος ἐστί . . . οὕτως καὶ ἄλλας ἀρχὰς δεῖ πλείους εἶναι, δι’ ὧν ἅπαν λήψεται τὸ σῶμα τὴν τροφήν, ὥσπερ ἐκ φάτνης, ἐκ τῆς κοιλίας καὶ τῆς τῶν ἐντέρων φύσεως. τὰ μὲν γὰρ φυτὰ λαμβάνει τὴν τροφὴν κατειργασμένην ἐκ τῆς γῆς ταῖς ῥίζαις (διὸ καὶ περίττωμα οὐ γίνεται τοῖς φυτοῖς⋅ τῇ γὰρ γῇ καὶ τῇ ἐν αὐτῇ θερμότητι χρῆται ὥσπερ κοιλίᾳ), τὰ δὲ ζῷα πάντα μὲν σχεδόν, τὰ δὲ πορευτικὰ φανερῶς, οἷον γῆν ἐν αὑτοῖς ἔχει τὸ τῆς κοιλίας κύτος, ἐξ ἧς, ὥσπερ ἐκεῖνα ταῖς ῥίζαις, ταῦτα δεῖ τινι τὴν τροφὴν λαμβάνειν, ἕως τὸ τῆς ἐχομένης πέψεως λάβῃ τέλος. ἡ μὲν γὰρ τοῦ στόματος ἐργασία παραδίδωσι τῇ κοιλίᾳ, παρὰ δὲ ταύτης ἕτερον ἀναγκαῖον λαμβάνειν, ὅπερ συμβέβηκεν⋅ αἱ γὰρ φλέβες κατατείνονται διὰ τοῦ μεσεντερίου παράπαν, κάτωθεν ἀρξάμεναι μέχρι τῆς κοιλίας.
And just as the mouth is a channel for the unprocessed nourishment, . . . so there must also be many other sources, through which the entire body will take its nourishment, as if from a trough, out of the stomach and the nature of the intestines. Plants take their nourishment already worked up out of the earth with their roots (wherefore plants do not produce waste products; for they use the earth and its heat like a stomach), but just about all animals, and obviously so those which have locomotion, have, like an earth inside them, the vessel of the stomach, out of which, just as plants with their roots, the animals must take the nourishment with something, until it reaches the end of its digestion. For the production of the mouth transmits to the stomach, from which it is necessary that something else pick it up, which is what happens; for veins stretch all throughout the mesentery, starting from below [and stretching] up to the stomach.
There are two complementary systems at work here. In the first, the food enters the mouth and is successively worked upon and altered by the organs of the digestive tract. When it exits the stomach and enters the jejunum at the beginning of the small intestine, Aristotle tells us that the food here ‘changes and is no longer still fresh nor is it yet excrement’.Footnote 20
At this point the veins from the mesentery somehow siphon off the useful product and usher it into the second system, the vascular, while the remaining solid waste continues down the first, digestive, system and out the other side. Aristotle declines to describe the precise mechanism of transference of the nutriment from the intestines to the veins in Parts of Animals, beyond reiterating the roots simile and underscoring that it is the responsibility of the mesentery, which is ‘full of many densely packed veins, which stretch from the intestines to the great vein and the aorta’.Footnote 21 For the details, he refers us to On Generation, which does not address it, and On Nutrition, which we do not have. As a result, we are solely dependent on the brief description in On Sleep (Passage 7) (Somn. 456a34–b6):
τροφὴ δ’ ἐστὶ πᾶσιν ἡ ἐσχάτη . . . ἡ τοῦ αἵματος φύσις . . . τόπος δὲ τοῦ αἵματος αἱ φλέβες, τούτων δ’ ἀρχὴ ἡ καρδία . . . τῆς μὲν οὖν θύραθεν τροφῆς εἰσιούσης εἰς τοὺς δεκτικοὺς τόπους γίγνεται ἡ ἀναθυμίασις εἰς τὰς φλέβας, ἐκεῖ δὲ μεταβάλλουσα ἐξαιματοῦται καὶ πορεύεται ἐπὶ τὴν ἀρχήν. εἴρηται δὲ περὶ τούτων ἐν τοῖς Περὶ τροφῆς.
The final form of nutriment for all creatures is the nature of blood . . . and the location of the blood is the veins, and the source of those is the heart . . . and when the nutriment enters the receptive parts from without, an exhalation comes about from it into the veins, and there, altering, it sanguifies and is carried to the source. There is discussion about this in On Nutriment.
Two interesting words emerge from this passage. First, the useful nutriment as it first passes into the vascular system Aristotle here describes as an ἀναθυμίασις, or ‘exhalation’, the word he uses for it repeatedly.Footnote 22 In the Meteorologica he explains that (Passage 8) (Mete. 359b28–34)
ἔστι γὰρ δύ’ εἴδη τῆς ἀναθυμιάσεως . . . ἡ μὲν ὑγρὰ ἡ δὲ ξηρά⋅ καλεῖται δ’ ἡ μὲν ἀτμίς, ἡ δὲ τὸ μὲν ὅλον ἀνώνυμος, τῷ δ’ ἐπὶ μέρους ἀνάγκη χρωμένους καθόλου προσαγορεύειν αὐτὴν οἷον καπνόν⋅ ἔστι δ’ οὔτε τὸ ὑγρὸν ἄνευ τοῦ ξηροῦ οὔτε τὸ ξηρὸν ἄνευ τοῦ ὑγροῦ, ἀλλὰ πάντα ταῦτα λέγεται κατὰ τὴν ὑπεροχήν.
there are two types of exhalation, the moist and the dry; the former is called vapour, the latter, as a whole, lacks a name, but, using the part for the whole, one must call it a kind of smoke; and it is not possible for the moist to exist without the dry, nor the dry without the moist, but all these [combinations] are spoken of [as moist or dry] according to which predominates.Footnote 23
This characterization suggests that, as it is heated in the process of digestion, the chyle in the intestines emits a steamy and particulate emanation that rises, in accordance with its hot, vaporous and smoky nature, into the veins and thence continues its upward path towards the heart.
The second, potentially more troubling, word is ‘sanguifies’ (ἐξαιματοῦται). Though it is not at all inconsistent to say that it changes in some way on this next step in its digestive journey, the exhalation from food has absolutely no business turning into blood in the veins before it gets to the heart.Footnote 24 Aristotle is consistently and unequivocally clear about the fact that blood originates in the heart and nowhere else. Indeed, in the same treatise, only a few pages after the quote we are considering, he reaffirms: ‘the source of all the blood, as has been said both here and in other places, is the heart.’Footnote 25 The word he uses here, ἐξαιματόειν, becomes the standard word in later medical theory for the manufacture of blood by the liver, hence its typical translation as ‘become blood’ or ‘sanguify’, but this is the only surviving attestation of it in Aristotle and indeed in any author before the first century a.d. There appear to me to be three different ways of dealing with this problem. The first is to assume, as some have done, that the verb here should be translated more along the lines of ‘take a step in the process of becoming blood’ rather than ‘actually become blood’.Footnote 26 The conjunction with the participle ‘changing’ (μεταβάλλουσα) does make this tempting, but this is by no means a standard use of the word and, for an apparently newly coined term, is asking the reader to do what is perhaps an unrealistic degree of interpretation. The second possibility is to assume that this passage is the interpolation of a later editor of the text. The next attestation of ἐξαιματόειν after this one is not until Philo Judaeus, the Jewish philosopher living in Alexandria at the turn of the first century a.d.; he uses it in the same way Galen does—to describe the blood-creating capability of the liver.Footnote 27 The term appears as well, with the standard later meaning, in a discussion in the Anonymus Londiniensis papyrus of the doctrines of the later Hellenistic author Asclepiades.Footnote 28 All this could conspire to suggest that the word was first coined in the flourishing of medical activity in Hellenistic Alexandria and, thus, that a Hellenistic editor of the Parua Naturalia is responsible for some entrenched interpolations in this section of the treatise.Footnote 29 The third possibility is to doubt that the word carries the same connotation here that it comes to have in later authors. Aristotle could simply be using the prefatory ἐκ- to indicate a complete or intensified degree of the verb αἱματόειν, which was used quite commonly in the fifth and fourth centuries b.c., from history to tragedy to comedy, to mean ‘make bloody’ or ‘stain with blood’.Footnote 30 In this model, the passive intensified form used here would likely best be rendered: ‘is covered utterly with blood’ or ‘is drenched in blood’. In other words, as would be consistent with its entrance into the vascular system, the exhalation from the food becomes incorporated into the bloodstream.
Regardless of how precisely one chooses to interpret the passage above, it is quite clear that the exhalation from the food has entered the vascular system and, thus, that it has entered the blood, for which the vessels serve everywhere as containers.Footnote 31 From here, the food must make its way to the heart in order to achieve its final concoction, and the heat inherent in the nutritive exhalation appears to facilitate this upward rise. Thus the blood with which the partially digested food particles are now saturated, agitated and heated by this intruding exhalation, will have to return to the heart via both the aortic system and the great vein system. Yet, the immediate next step on the journey for this food exhalation that Aristotle describes is not in the heart but in the head.
Aristotle describes the role of the brain in digestion in several different texts.Footnote 32 In Parts of Animals he introduces the analogy of evaporation leading to rain to explain why the coolness of the brain leads to the production of phlegm and nasal congestion. He says (Passage 9) (Part. an. 652b36–653a2, 10–16):
ἀναθυμιωμένης γὰρ διὰ τῶν φλεβῶν ἄνω τῆς τροφῆς τὸ περίττωμα ψυχόμενον διὰ τὴν τοῦ τόπου τούτου δύναμιν ῥεύματα ποιεῖ φλέγματος καὶ ἰχῶρος . . . ποιεῖ δὲ καὶ τὸν ὕπνον . . . τοῦτο τὸ μόριον . . . καταψῦχον γὰρ τὴν ἀπὸ τῆς τροφῆς τοῦ αἵματος ἐπίρρυσιν . . . βαρύνει τε τὸν τόπον (διὸ τὴν κεφαλὴν καρηβαροῦσιν οἱ ὑπνώσσοντες) καὶ κάτω ποιεῖ τὸ θερμὸν ὑποφεύγειν μετὰ τοῦ αἵματος.
after the food has risen up as an exhalation through the veins, the residues, cooling down because of the power of this place [the brain], cause discharges of phlegm and ichor . . . And this region also causes sleep . . . for, cooling down the influx of the blood from the food . . . it both weighs down its part of the body (wherefore sleepy people have heavy heads) and makes the heat recede downwards, together with the blood.
In On Sleep he refers to this same event repeatedly, discussing it, at various points, in similar terms. There he tells us that sleep ‘comes about from the exhalation attendant on food’, which, because of its very nature as an exhalation, is ‘necessarily thrust up to a certain point, and then wheels about and changes course, as if in a narrow strait’.Footnote 33 Because heat has a tendency to rise, he tells us, the food exhalation—and, presumably, its attendant blood, the substance being here described as ‘both liquid and particulate’—goes right up to the head ‘through the veins’, before it ‘sinks downwards’ ‘upon the primary aisthētērion’, that is, the heart.Footnote 34
The migratory substance in question here is interchangeably characterized in Passage 9 both as a food-exhalation and as blood, confirming the preceding analysis that the as-yet-unconcocted nutritive matter is travelling in a mobile stream of blood.Footnote 35 Indeed, Aristotle says explicitly in On Sleep that ‘after the influx of food, the blood gets mixed up’, indicating clearly that the movement of the nutritive exhalations effects a corresponding movement of blood.Footnote 36 Given the arrangement of the vascular system, this blood-nutriment mixture cannot get to the vessels of the head without going through the heart on the way. Since the nutritive matter that reaches the head continues to be described as an exhalation, rather than as fully blood, we must infer that it traversed the heart in this initial pass without lingering long enough to be fully concocted into blood. Indeed, given the turbulent upward velocity that Aristotle ascribes to it, it is an open question whether any significant quantity of this food-laden blood would have time to get from the two side-chambers, to which the main vessels connect, into the central chamber, which, as I will discuss shortly, seems likely to be the special blood-producing region. Even if some small part of the new food exhalation managed to achieve full concoction on this initial pass, it certainly appears that the majority of it continues, unfinished, straight up to the head.
Once in the head, the blood-food mixture undergoes a change. In Passage 9 above we learned that the cooling power of the brain chills the residues, causing ‘discharges of phlegm and ichor’. The parallel discussion in On Sleep offers more details: ‘the residue-like exhalation is condensed into phlegm, but the nourishing and not unwholesome exhalation, condensed, is carried down.’Footnote 37 Given the parallel situations in these two passages, it seems safe to conjecture that the cooling process of the brain causes the phlegmy residues in the food-blood mixture to collect and filter out in a discharge of phlegm. This condensation of the residual material causes the remaining, beneficial, parts of the nutritive exhalation, having been thus filtered of phlegm and themselves ‘condensed’, to then flow down as a separate discharge of ichor, which Aristotle defines in History of Animals as ‘unconcocted blood’ and in Parts of Animals as ‘the part of blood that is watery on account of not yet having been concocted or of having decomposed’.Footnote 38 Thus the trip to the brain seems to have materially altered the nutritive exhalation in such a way that it is now purer and ready to undergo the final step of concoction in the heart, having, in the process, more effectively homogenized it with the blood that is carrying it—it is no longer an exhalation mixed with blood but is now ichor, a more integrated, albeit unfinished, element of the bloodstream.
After this filtering process in the head, Passage 9 above indicates that the now more fully integrated nutritive matter goes ‘downwards with the blood’. Given the vascular arrangement, the only plausible destination for this downward flow, which has now lost its effervescent degree of heat, is the heart, and the passages quoted above from On Sleep indeed confirm that it flows to the ‘primary aisthētērion’. Thus the blood (which has definitionally already been there at least once, at the moment of its formation) and the now-purified food exhalation that it is transporting are making their second trip to the heart on this digestive journey so far.
The very process of digestion and blood creation, then, involves the flow of already created blood into and out of the heart. But even outside this unusually turbulent period, when ‘the blood [becomes] mixed up after the intake of food’, there seems to be a constant, low-level movement of blood, not-yet-blood and other materials into and out of the heart and up and down the vascular system.Footnote 39 First of all, despite its immediate return to the heart, all of the ichor distilled by the brain does not achieve final concoction on that trip. The ichor, as part of the blood, appears throughout the body. Children, for example, who crop up throughout the corpus as creatures who eat excessively and whose heads are full of food and thus disproportionately large and heavy, have blood throughout their bodies that is especially ‘ichor-y’ and abundant.Footnote 40 In pathological situations, where a person's blood has become ‘excessively liquid’ and ‘ichor-y’, it is even able to exit the pores in a ‘blood-like sweat’.Footnote 41
The fact that this not-completely-concocted proto-blood is able to travel in the body comingled with the completed blood is one way to explain some otherwise puzzling passages in which other organs take part in digestion and in the concoction of blood. In On Youth and Old Age, Aristotle tells us that ‘all the parts of the body work on and concoct the food with their natural heat, but most of all the most authoritative part [the heart]’.Footnote 42 In Parts of Animals he specifically singles out the digestive roles of the liver and the spleen, which, again, because of the arrangement of the vascular system are not in a position that could be logically expected to receive all of the initially absorbed exhalation prior to its arrival in the heart.Footnote 43 Further, it is because they are full of blood, and therefore hot, that they are able to assist in concoction and this blood, as in all organs except the heart, must be contained in the vessels, which, in this model, is also the only plausible location for the food that they are working on—indicating, once again, that the incompletely concocted food (which must eventually go to the heart) is contained in blood (which must eventually return there to deliver it).Footnote 44
Blood, then, in the final step of digestion (for this particular batch of food that we are following, at least) will have to go to the heart once again in order to facilitate the final sanguification of the proto-blood it is transporting. Nor is this a hypothetically necessary but unsubstantiated claim. Despite the strong words in Parts of Animals with which we began, Aristotle suggests influx into the heart as a commonplace in several passages across the corpus. In Parts of Animals, for example, he mentions that pain, pleasure and sensation proceed from the periphery to the heart, and indicates that it is the blood that facilitates this movement.Footnote 45 Most emphatically, he implies in On Sleep that the heart ‘receives’ blood from the two main vessels and, in On Dreams, says explicitly that during sleep ‘the majority of the blood goes down to the source’.Footnote 46
The obvious next question is: what precisely happens in the heart? How and where does the final change take place and how can the seemingly simultaneous inflow and outflow of blood and nutrition be reconciled? As to the how and where of the change, the descriptions of the heart in both Parts of Animals and On Sleep indicate that the central chamber is the source within the source—it is not a large leap to infer that its special balance of heat and its markedly important place in Aristotle's theory of locational hierarchy allow it to act on the nutriment in the special way required to produce blood. As to the traffic problem posed by influx and efflux along the same channels, the fact that the nutrition is mixed in with the blood as an integrated part of the bloodstream obviates the need for physiologically improbable situations like the situation Boylan posits with his suggestion that ‘perhaps the [uena caua] has two channels, or, more probably, that blood and nourishment are mutually insoluble, so that, like oil and water, they coexist without mixing’—a theory which has to do the arduous work of allowing these two insoluble liquids to somehow be flowing simultaneously through each other in opposite directions.Footnote 47 Rather it seems more probable to imagine a uniform flow into and out of the heart which consists of both fully and partially concocted blood, with the unfinished blood gradually achieving the final step of concoction on its various passes through the heart.Footnote 48
The closest Aristotle comes to discussing the mechanics of this process is in the treatise On Respiration. Here he is describing the difference between ‘pulsation’ (σφύξις), which accounts for the perceived heartbeat, and ‘respiration’ (ἀναπνοή), which is a movement of the heart that causes the inflation of the thoracic cavity and thus the inhalation of air into the resulting vacuum in the lungs. Pulsation he particularly equates to the pulsating pain of a pathological swelling or to the boiling of water. In the former, the contained liquid cannot escape and eventually boils down and putrefies; in the latter, the boiling water simply overflows its container. In the heart (Passage 10) (Resp. 480a2–7),
ἡ τοῦ ἀεὶ προσιόντος ἐκ τῆς τροφῆς ὑγροῦ διὰ τῆς θερμότητος ὄγκωσις ποιεῖ σφυγμόν, αἰρομένη πρὸς τὸν ἔσχατον χιτῶνα τῆς καρδίας. καὶ τοῦτ’ ἀεὶ γίνεται συνεχῶς⋅ ἐπιρρεῖ γὰρ ἀεὶ τὸ ὑγρὸν συνεχῶς, ἐξ οὗ γίνεται ἡ τοῦ αἵματος φύσις. πρῶτον γὰρ ἐν τῇ καρδίᾳ δημιουργεῖται.
the heat-induced swelling of the constantly in-flowing moisture from the nutriment causes pulsation, as it rises towards the outer layer of the heart. And this is always happening continuously. For the liquid from which the nature of blood comes about always flows continuously in. For it [blood] is first created in the heart.
This is coupled with the comparatively slower process (matched to observable respiratory rate rather than heart rate) of respiration, which (Passage 11) (Resp. 480a16–20)
[ἡ δ’ ἀναπνοὴ] γίνεται αὐξανομένου τοῦ θερμοῦ ἐν ᾧ ἡ ἀρχὴ ἡ θρεπτική. καθάπερ γὰρ καὶ τἆλλα δεῖται τροφῆς, κἀκεῖνο, καὶ τῶν ἄλλων μᾶλλον⋅ καὶ γὰρ τοῖς ἄλλοις ἐκεῖνο τῆς τροφῆς αἴτιόν ἐστιν. ἀνάγκη δὴ πλέον γινόμενον αἴρειν τὸ ὄργανον.
comes about with the increasing of the heat in which the nutritive source resides. For just as all the other parts also require nourishment, so does it [the heart], and more than the others. For this is the cause of nourishment for the others. And it is necessary that the organ swell as this [nourishment] becomes more abundant.
The swelling of the heart that causes respiration is cooled (and therefore deflated) by the cool air pulled in through the opening of the pulmonary cavities, thus causing cardiac contraction, which in turn mechanically leads to exhalation.
The differences between these two motions of the heart are puzzling, given that they seem to result from the same cause yet proceed at markedly different rates. The best way to imagine it is perhaps that the influx of food-laden blood expands under the influence of the heat of the heart, pushing out the heart in a pulsation. Though the heart is not open like a pot of boiling water, unlike a tumor, it does have egresses, namely the great vein and the aorta. One can imagine an estuary-like situation there: as the pressure of the cardiac contents builds up, it expands the walls of the heart and some of it manages to overcome the influx of fresh food-laden blood through these openings and create a countervailing rush of exiting, fully processed blood. Some of this new blood would then be brought right back in with the ‘ichor-y’ or food-laden blood, but, depending on how recently the person has eaten and where the prevailing movements in the bloodstream are heading, some of it might get swept up to the head or fall down to the lower organs. These releases of blood, however, can only account for small and temporary relief and the heat, the liquid and the hot evaporations in the heart continue to expand, until, at last, the respiratory movement is achieved, cooling down the boiling turmoil and dramatically lessening its volume.
Within these violent and countervailing movements, the central chamber of the heart maintains an incongruous peace. As we saw in Passage 4 above, Aristotle describes it as ‘calm’ and containing blood that is ‘pure and intermediate as to quantity and heat’. This is the place in the body where the heat—that vital quality of life in the Aristotelian model—is at its most perfect degree. This is also very likely the site of the primary aisthētērion, where the soul interacts most intimately with the body. These two qualities allow for the perfect conditions within which to convert the heavily purified and concocted nutritive matter mixed in with the blood into actual blood itself. Aristotle offers no clear or definite description of this process. My own suspicion is that we can lean fairly hard on the boiling simile in On Respiration—‘[pulsation] is similar to boiling; for boiling comes about when moisture has been aerated by heat’.Footnote 49 It does not seem out of the question that the final concoction of blood from proto-blood involves an addition of the mysterious but powerful inborn pneuma, which often seems to operate within it.Footnote 50
Once the blood, whether newly or previously finalized, manages to escape the tumultuous back-and-forth of the heart, it is free to wander through the vascular system. What propels it on this journey is an open question. Aristotle seems largely uninterested in staking out a position in what will, not long after his death, become one of the more hotly contested debates in the field. It is certainly not out of the question that he thought that the contraction of the heart offers enough propulsion to send the blood far out into the vascular system, much as the correlated contraction of the lungs propels the air out in exhalation. It is also not impossible that the pneuma with which the blood is imbued has some sort of motive role, though Aristotle may seem to suggest elsewhere that this is not the case.Footnote 51 The most likely candidate seems to me to be the heat.Footnote 52 As we saw already in the earlier stages of digestion, the heat of the nutritive exhalation drives it up to the top of the body, where, like evaporated water forming rain clouds, it cools, becomes heavy and falls back downwards. A similarly heat- and weight-based framework might be enough to drive the remaining periods of blood flow. Indeed, we are told in On Sleep that, after digestion is resolved, the pure, thin blood goes to the upper parts of the body, whereas the heavier, turbid blood goes down.Footnote 53
FLESH AND THE RESIDUES: THE CASE OF SEMEN
The final fate of the blood is to become flesh or a residue.Footnote 54 Viewed from a chronological standpoint, there are two different kinds of residue.Footnote 55 The first are those which are removed prior to blood formation—these are the elements of the food that were unsuited to be assimilated to the animal's body, including feces, urine, phlegm and bile.Footnote 56 The second group of residues are created from excess blood—the result of eating more food than is strictly required for the maintenance of the body. These include hard fat, soft fat (suet), bone marrow, breast milk, menstrual blood and semen.Footnote 57 This latter category opens the window for surplus blood to linger in the vascular system, allowing it to conduct the nutritive matter in the various ways we have already considered. Some of these post-blood residues, especially fat and bone marrow, might reasonably be created at the end of the line: once it is clear that the organ or body part to which the blood has arrived has consumed enough blood to sustain itself, the remainder gets further concocted on the spot.Footnote 58 The case of semen in particular, however, suggests a wide-ranging mobility for this excess blood.
Semen, to reiterate, is a residue formed by the further concoction of excess blood.Footnote 59 In viviparous creatures, at any rate, it is created on demand during the act of copulation.Footnote 60 Aristotle is quick to deny that semen is drawn from all parts of the body—indeed, he spends a considerable portion of Generation of Animals rebutting the theory of pangenesis—but naming blood as the material from which semen is made is appealing precisely because blood is potentially all parts of the body. Thus semen retains blood's capacity to assimilate to any part of the parent's body, allowing for the generation of all the various parts of the fetus as well as the resemblance of the offspring to the parents.Footnote 61 Like the other natural residues, semen has a special location in which it collects, namely the testicles; however, the amount of blood that collects in the testicles to be further concocted into semen appears to be entirely dependent on the needs of the rest of the body.Footnote 62 If the diet of an animal results in too much blood being converted into fat, it has less available for semen and becomes increasingly infertile; if an animal is extremely large, it requires a great deal of nutriment to sustain itself and thus has very little leftover to turn into semen; if the animal is growing rapidly, it likewise needs to allocate all of its nutriment to its parts rather than to generation.Footnote 63 In contrast, the weakness that the animal feels immediately after the creation and release of semen—a weakness that he elsewhere says is felt not just in the generative organs but throughout the whole body—is ‘manifest’ and is equivalent to and results from the same cause as the weakness experienced at the loss of too much blood: the animal has been deprived of nutriment.Footnote 64 In short, the amount of blood available to become semen at any given time and the amount of blood used in all the other parts of the body seem to be directly interrelated, suggesting that the body has the ability to naturally reallocate excess blood as needed, in turn requiring that the blood move in ways other than a simple radiation out from the heart.
CONCLUSION
It is dangerous to take Aristotle too closely at his word in the passage from Parts of Animals with which I began. His statement that ‘blood is conducted out of the heart and into the veins, not from elsewhere into the heart’ is entirely about the origin of blood, not about its subsequent behaviour. Nutriment can only achieve full blood status in the heart, thus blood does not originate elsewhere and then go to the heart. This very claim, however, necessitates the various digestive peregrinations outlined here and ensures that, once it has been created, the blood travels widely and in many different directions in the vascular system, including repeated return trips through the heart. In short, Aristotle does not describe the simplistic anti-circulatory system that is so often ascribed to him. Rather he presents a fully functional and coherent system of blood flow, consistent with his understanding of human anatomy and physiology, positional hierarchy, and cardiocentrism. There is sufficient evidence to allow us to understand it on its own terms, without reference to any of the theories that post-date it.
