by Julie C.
One of our most popular demos at the Exploratorium is the cow eyeball dissection.
Before we cut open the eye, we often ask for predictions about what we’ll find inside. Will it be hard or soft? Full or empty? Fluid or solid? Will it be bloody or clear or melanin-colored? We get all sorts of interesting predictions which flow into more questions about eyes.
The other day, I got a question from an interested visitor: “Does a horned toad have blood inside its eyes? Because horned toads can shoot blood from their eyes to scare predators.” I didn’t know the answer and we discussed some possibilities. We thought about tear ducts, blood within the eye, and how fluid moves in the eye.
Here’s the answer: Horned Toads (actually lizards) shoot blood from their eyes by “restricting the blood flow leaving the head, thereby increasing blood pressure and rupturing tiny blood vessels around the eyelids” (Wikipedia). Some bugs and snakes also autohaemorrhage to protect themselves from predators. There have been cases of humans “crying blood”. Human haemolacria isn’t a defense reaction but is thought to be a symptom of another disease, an injury, or tumor.
I do not recommend trying to cry blood, rupturing your tiny blood vessels, etc! Instead, check out the exhibit Blood Vessels of the Eye in the Exploratorium’s Central Gallery or try this version of the exhibit at home. And if you’re interested in cow eyeball dissection, here are how-to instructions!