Zen skulls were a Tesshu trademark. Naturally, a skull reminds us of the transitory nature of life and of all pleasure. However, at the same time, from the Zen standpoint, it reminds us to "Live completely, die completely!" This skull is that of a great beauty, and now she is just a pile of bones. But nothing is wrong with that fact—that is the way life (and death) are meant to be.
In terms of output and quality, Tesshu was the greatest of samurai Zen artists. He was a master of the sword, the brush, and Zen and, although he died at the young age of fifty-two, he produced well over a million pieces of Zen art. He ranks as one of the finest calligraphers in Asia, and his work is treasured.
See The Sword of No-Sword by John Stevens (Shambhala, 2001) for a complete biography of this martial-arts hero.