Dos Santos' third leg, which was attached to his lower spine by only a small band of tissue, was a composite of two legs, having eight toes, two heels, and two tibiae and femora. He had neither sensation nor active movement in the extra leg. The knee was completely ankylosed and immobile; dos Santos claimed this was due to a Portuguese doctor having broken and reset it when dos Santos was a year old, in order to make the leg less obtrusive. The leg could be concealed easily by strapping it tightly to his right thigh. Thus, dos Santos was not at all encumbered by the dangling appendage, and he walked, ran, and rode horses with ease.

It was not dos Santos' three legs that most excited medical men, but his double genitalia. He possessed two functioning penes and three scrota, the outer two of which each contained a single testis. Dos Santos claimed that the central scrotum had also contained a pair of fully-formed testes, but that these had retreated into his abdomen when he was ten years old. Wrote the photographer C.D. Fredericks, creator of the only known photograph of dos Santos, in 1865, "...the sight of a female is sufficient to excite his amorous propensities. He functionates with both of the penes, finishing with one, then continues with the other." He urinated and achieved erections with both penes simultaneously. In fact, since his fame coincided with the career of Blanche Dumas, the three-legged courtesan, it was rumored that the two of them had an affair!