Tristram's about to make a brilliant observation: writing is just another name for conversation. Just as you have to use judgment about what to say when you're talking, you have to leave something to the reader's imagination. Tristram lets the reader imagine what Dr. Slop, dripping with mud, says; as well as the story Obadiah tells. Tristram also lets the reader imagine that Mr. Shandy has gone upstairs to check on Mrs. Shandy and that, in the meantime, Dr. Slop gets cleaned up. As he cleans himself, he realizes he's forgotten something: his instruments.
Off goes Obadiah, to fetch the bag of instruments.