"Joining animals together, known as parabiosis, has a long history in science. In the 19th century, French scientists connected the blood vessels of two rats. To prove that the rats shared a circulatory system, they injected belladonna, a compound from the deadly nightshade plant, into one of the animals. The pupils of both rats dilated."

https://www.nytimes.com/2023/07/27/health/mice-blood-aging.html