what differences does it made when one's stomach is full ? In three men in a boat

The narrator thinks of this statement after having a nice supper at the Picnic Point, along with George and Harris. They all were very eager to have that meal and feel really contented after having it. It is this feeling of contentment that makes the author believe that one feels immensely good and satisfied with himself and the world, when he has a full stomach. He says that though people believe that only a clear conscience can make them very happy and contented, a full stomach can give them the same level of satisfaction. He feels that after having a hearty meal one naturally feels forgiving and generous. For instance, before the supper, all three of them were quarrelsome and snappy and ill-tempered, but after it, they sat and smiled at one another, feeling happy, contented and compassionate. 

He feels that the digestive organs of a person dominate his intellect. He cannot work, cannot think, unless his stomach wants him to do so. It dictates to him his emotions and passions. As pointed out by him, after eggs, it is only the stomach that instructs the brain to work, after a thick slice of meat to sleep, after a cup of tea to rise and show strength, after hot muffins to be dull and soulless and after tonic to grin and enjoy.

He further comments that keeping in mind such strong influence of our stomach on our behavior, we should choose our food carefully. Good choice in food can help us become a good citizen, a loving husband, a tender father and a noble, pious man. 

I hope you find this answer helpful. Please post more questions on the forum to be assisted by our team.
Thank you.