The Gemara talks about metzitzah for health reasons. The Gemara also mentions bathing the baby in warm water for health reasons. Poshu pshat is there is no difference among the two, and just like we no longer bathe the baby in warm water because we don’t believe it to necessarily be healthy for the baby, we shouldn’t have to do metzitzah either if we don’t believe it’s healthy for the baby.
Some poskim however claim, that for reasons I don’t understand, metzitzah has become part and parcel of the mitzvah while bathing has not, therefore metzitzah must be done even if it no longer has known health benefits.
My question is, to the best of my knowledge, no where does the Gemara or the early rishonim say that metzizah has to be done directly with the mouth. Metzitzah just means to suction out the blood. It was done then directly with the mouth because their keilim weren’t sophisticated enough. What is the source that when the Gemara says metzitzah it must be done directly with the mouth and that the suctioning cannot be done with a tube? From the health perspective, it seems that both would accomplish the same goal, so why aren’t both valid ways of fulfilling this detail?