Is there a specific name for the seams of shirts that follow from under each breast to the bottom of the shirt?

Those "seams" are actually called darts. Darts differ from seams because seam are used to join separate parts of fabric together. Darts are used to add shape to a garment like to taper a shirt from the bust to the waist. Darts are also used to customize the fit of a garment like to add shape to a straight or boxy shirt.