请将下面这段话翻译成英文:
土司制度
土司制度是中国元代以来中央王朝对西北、西南边疆的统治办法,也是维系中国多民族统一国家的重要制度之一。土司制度有利于将中央很难派官吏直接统治的地区纳入到统治范围,对统一的多民族国家有好处;但在土司统治下,土地和人民都归土司世袭所有,土司各自形成一个个势力范围,造成分裂割据。
参考译文
Tusi System
The Tusi System was a ruling method used by the central government of the northwest and southwest frontiers of China during the Yuan Dynasty. It was one of the most important systems in maintaining China as a unified multi-ethnic country. Tusi was a hereditary ruler appointed by the central government. The Tusi System was conducive to ruling areas where it was difficult for the central government to appoint officials to directly administrate. The underside of the system, however, was that all land and people living on it were possessed as hereditary property by the Tusi who, forming his own sphere of influence, took de facto control of the area.