Several physiologists who have attempted to explain the cause of superfœtation have supposed that in such cases the uterus is virtually _double_; _Morgagni_ informs us, that _Catti_, the Neapolitan anatomist, was the first to observe this phenomenon, and that it is owing to a strong membrane which so divides the uterus, that the mouth of a fallopian tube corresponds with each of its cavities; and he farther states, that this strange structure was found combined with a corresponding division of the vagina; _Valisnieri_[426] also met with a double uterus, and a double vulva; the same malformation has been noticed by _Littre_,[427] _Bauhuin_,[428] _Eissenmann_,[429] _Haller_,[430] and by _Rhoederer_; this latter physiologist in a letter, from Strasburgh, preserved among the Sloane manuscripts, says, “We have got here a great curiosity, viz. a woman body of eighteen years of age, who has the natural parts externally well formed, but internally two vaginæ, each with its _hymen_, to which responds also an uterus duplex having two orificia, each of ’em hanging in its proper vagina, that in such a manner there is quite a double system of generation, and if she had been living a superfœtation could have been formed.” _Sabbatier_ says that he believes in the possibility of superfœtation, and that the above formation will explain its occurrence; an opinion which is sanctioned by _Gravel_[431] and _Teichmeyer_;[432] _Duffien_ also observes, “_Cette double matrice sert très bien a expliquer la superfœtation_.”[433] In quadrupeds superfœtation very commonly occurs, and it has been explained by supposing that the uterus of these animals is divided into different cells, and that their ova do not attach themselves to the uterus so early as in the human subject, but are supposed to receive their nourishment for some time by absorption; hence the os uteri does not close immediately after conception; for a bitch will admit a variety of dogs while she is in season, and will bring forth puppies of these different species; thus, it is common for a greyhound to have in the same litter, one of the greyhound kind, a pointer, and a third or more, different from both.[434]