How early Greek philosophers used animal dissection

We’ve explored in previous posts how Greek thinkers shied away from using human dissection: there was an extremely powerful taboo that discouraged even going near a human corpse, but there was no such prohibition about animals. The slaughter of animals for religious purposes meant that the Greeks were used to working with the bodies of animals, and, at some point in the 5th century BC, Greek thinkers began to use this animal dissection as a way of drawing conclusions about human internal anatomy.

Without the use of human dissection, the interior of the body was a mystery. The Greeks gained an awareness of some of the larger structures, such as the intestines, from observing war wounds. But even in the medical practice of the time, surgery was strongly discouraged.

Greek doctors had developed a strong sense of the limitations of their own practices. Cutting people open in, say, the 5th century BC was a recipe for problems, not least of all were infections and the fact that it just was not obvious to any would-be surgeon what to do with the insides of the body.

This led to a loop: Greek doctors stayed away from working on the insides of living humans, and so they reinforced their ignorance of the human interior, which gave them more reason to stay away from working inside a human body.

Animal dissection became a natural candidate for how to study the inside of a human body. The general practice of inferring things about the human body from animal bodies is called comparative anatomy. Scientists continue this practice today when they publish speculations about, say, human metabolism by having studied rat metabolism, and this practice was invented by the early Greek philosophers in the 5th century BC.

In the Histories, Herodotus (ca. 484 - 424 BC) reports that cows in Scythia have large amounts of bile in them and that you can confirm this by “dissecting a Scythian cow” (IV.58). This report is not so striking in itself: Herodotus thinks that the bile is made from all the grass that cows eat, and Scythians live on a plain with a lot of grass.

But what is striking is how casually he reports conclusions from animal dissections. This reflects the fact that, at some point in Herodotus’ century, the Greeks started conducting animal dissections.

Hippo (5th century BC) is the earliest thinker whom I can find who used animal dissections specifically for the purposes of comparative anatomy. Censorinus (3rd century AD) attributes this experiment to Hippo:

“Hippo […] believes that semen flows from the marrow and proves this by the fact that if, after the male livestock has been admitted to the females, the males are slaughtered, one does not find their marrow, since it has been exhausted” (D9).

Hippo tried to prove that marrow was the same thing as semen by conducting an experiment that showed that if you deplete an animal’s semen, you deplete its marrow. In the minds of many ancient Greek philosophers, the stuff at the inside of our bone, which we today would call ‘bone marrow’, was semen and the stuff that made up our brain. This facilitated a view, held by some, such as Plato (428 - 348 BC), according to which a man’s brain is connected to his skeletal system, which contains marrow, and also to his reproductive organs, which emit marrow during sex.

We don’t know what Hippo thought about all of this, but the results of his (alleged) experiment facilitated ancient Greek theorizing about how our brains and reproductive system are connected.

Another experiment is reported to us in the anonymous text On the Heart. The author says: dye water blue and then let an animal drink it, and then cut up open its body; we would find that its windpipe had been dyed blue (§2).

The author believes that this experiment shows that some of the water that we drink ends up in our lungs and travels down the windpipe. When we drink something, it doesn’t all go to the stomach.

The author has a very interesting reason for believing this matters. When our hearts pound in our chest, that’s because they’ve gotten hot. They need to be cooled down somehow. And our lungs exist as something like an ice pack or a cooling pad for the heart. Our overheating hearts press up against the lungs and get cooled down. What makes the heart cool? Well, partly air, but also partly the water we drink that has arrived in the lungs via the windpipe. So, the anatomy that animal dissections reveal (or allegedly reveal) shines some light on physiology.

On the Heart was written quite a bit later than Hippo and the dawn of comparative anatomy, probably in early 3rd century BC. We know this because it has detailed knowledge of things that were discovered only later. But the experiment that the text records seems to have been known earlier. That’s because Plato in the Timaeus says the exact same things about the windpipe, the lungs, and the heart. He doesn’t report to us how he knows them, but he seems to have gotten these beliefs from the same experiment that the author of On the Heart discusses.

So, for that reason, we should mark down the experiment in On the Heart as being earlier than the text itself.

So, if that’s the dawn of comparative anatomy and animal dissections, what comes later? Not much for a little while. Plato seems to rely on the results of animal dissections but never explicitly talks about dissections, from which we might infer that he heard about them being done but never did one himself.

Aristotle (384 - 322 BC), Plato’s student, is the real champion of comparative anatomy in antiquity. Consider what he says here:

“The inner parts of human beings are especially unknown, so that we must refer our investigation to those of the other animals that have a nature that is about the same” (History of Animals I.16 494b21-24).

Here, he shows an acceptance of the limitations of Greek knowledge of internal human anatomy and consciously resolves to study analogous organs in similar animals.

The history of comparative anatomy continues from there right through to the 21st century, as scientists even today conclude things about humans from studying other animals.