Pretty graphic imagery from Jesus here: Do you not see that whatever goes into the mouth, enters the stomach and goes into the sewers?
But these beginning verses are actually an answer to his disciples about the Pharisees, how they defile with what they say. Here is Jesus in full critique mode, he is critical of the blind adherence to the Law. Specifically he is alluding to the dietary practices for holiness, while spreading evil in words.
The mouth being...what? The gate between the inner and outer life. James talks of the tongue as the rutter of a ship, very small but it controls the entire vessel. The mouth, metaphorically, is the gate of our inner life, Jesus is saying that what goes in that gate soon is in the sewer. But what comes out of that gate is murder, adultery etc.
Silence is a good medicine. One of the Church Fathers, his name escapes me said: I've often regretted speaking, but I've never regretted being silent. Or as Mark Twain puts it: It's better to be silent and thought a fool than to speak and remove all doubt.
This is a problem I have, in the discernment meetings I just keep talking and talking. I get myself into trouble because I think I need to fill the space with my voice. The truth is I want to be still and quiet most of the time, my wife and co-workers would probably laugh at this notion. The meetings I've had are fundamentally artificial because of its structure: the one leading the group, with the power of decision, asking questions and waiting for responses. Then moving on to another question. Very little discourse. Conversation cannot happen when only one person is sharing themselves. I takes on a very extravegant feel, me sitting there and just vomitting all over the room...I need to watch that...