Blaschko's lines

This picture shows Blaschko's lines, which result from the migration of cells in an embryo.

Although they are normally invisible, they become visible in some people with skin disorders, as in this photo:

I learned this from Blaschko's Lines on Pharyngula, which has some more pictures and a very clear explanation of how this phenomenon works. The top image is from Dynamics of Cancer by Steven A. Frank; the bottom image is from Wikimedia Commons.

for SCIENCE!

In the 1800s, a surgeon named Edward Jukes came up with the technique of gastric lavage (stomach pumping). He demonstrated the efficacy of the technique by intentionally overdosing on laudanum, which contains morphine and codeine, then having his stomach washed out. He survived to write a somewhat strange book (for example, he recommended gastric lavage as a treatment for women who were prone to miscarriage).

I learned this from The Courageous - and Weird - Tradition of Self-Experimentation.