Artist's commentary
This is what you do to a sailor uniform!
Last night, I watched the "Trivia King"(http://www.tv-asahi.co.jp/zatsugaku/) TV program.
Apparently "the v-shaped opening at the front collar of a sailor uniform" was designed for seamen who might fall overboard and be at risk of drowning. It's meant so that they can easily tear off and discard their shirts so that they might more easily swim.