1. Laboratory of Veterinary Physiology, Kitasato University School of Veterinary Medicine, Towada, Aomori 034-8628, Japan
2. Present address: Laboratory of Veterinary Surgery, The University of Tokyo Graduate School of Agricultural and Life Sciences, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-8657, Japan
3. Laboratory of Veterinary Anatomy, Kitasato University School of Veterinary Medicine, Towada, Aomori 034-8628, Japan