History of Animals
The longest surviving fragment of Diogenes is that which is inserted by Aristotle in the third book of his "History of Animals". It contains a description of the distribution of the blood vessels in the human body. It is notable chiefly because "here we can read at first hand what in the case of the other Presocratics we learn only indirectly: an attempt to describe in scientific detail the structure and organization of the physical world."