1. Department of Anatomy, Showa University School of Medicine, Hatanodai, Shinagawaku, Tokyo 142, Japan
2. Departnient of Anatomy, Jikei University School of Medicine, Nishishinbashi, Minatoku, Tokyo 105, Japan
3. Department of Oral Anatomy, Showa University School of Dentistry, Hatanodai, Shinagawaku, Tokyo 142, Japan