|MadSci Network: Medicine|
The main reason why a newborn baby would smile while passing gas is relief. Before a babt passes gas, his lower colon and rectum are distended. After he passes gas, he feels relief because these organs are no longer distended. He expresses this by smiling. A second reason why a baby might smile is that the anal area is a very sensitive area. When gas (or other stuff) flows through the anus, the stimulation might feel good. In addition, the gas would be warmer than the surrounding air, and he might feel this warmth as comfortable. How this sensation might contribute to a baby's smile would depend on several factors, like how distended he was before he passed gas, the amount of sensory endings in the baby's anal and how sensitive he is to bodily stimuli.
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