中國自秦代以來，就曾以「犬牙相入」的原則劃定政區邊界，目的在維護高度中央集權制。其後陸續發展，至明代更進一步將「犬牙相制」的概念運用在軍事區劃與行政區劃的邊界劃分上。相鄰的都司、布政司邊界如鋸齒般咬合在一起，另有一些「軍事飛地」地處二都司交界，卻直隸於中央的五軍都督府。本文探討的潼關衛，地在河南、陝西交界，自洪武九年設衛，初隸屬於河南都司，永樂六年改隸北京留守行後軍都督府，北京留守行後軍都督府革廢後一度改隸後軍都督府，最後又改為直隸中軍都督府。其所屬一變再變，與其轄區遠離直隸地區，先天具備管理上的困難有關。這一類直隸衛所的配置，本是以君主專制為出發點，以支援中央、箝制地方為目的，然而隨著中央統治力量的衰弱，不但不能發揮應有的功效，反而必須仰賴地方，藉地方之力始能正常運作，可以說極大程度的見證了明代君主專制政權由盛而衰的過程。 「犬牙相制」的現象還體現在衛所軍屯與州縣民地的交錯上。明代潼關衛七十二屯分散在二州七縣，各州縣內部均呈現軍屯與民地犬牙相錯的局面。軍民雜處、軍強民弱的結果，屯軍不當侵蝕地方資源的情形屢有所見。入清以後，潼關廢衛改縣，其初猶為兵餉供應、屯糧徵收之便保持原潼關衛屯的完整性，然終究 難抵屯地分散兩省十一州縣所帶來的困擾，逐步縮編成僅掌管潼關、華陰兩境內屯地的潼關廳，明初以來以「犬牙相制」為目的建立的機制一步步邁向土崩瓦解的境地。

The principle of interlocking territorial administrative districts to demarcate the boundaries between different regions of the empire has existed in imperial China since the Qin Dynasty. The purpose of this system was to achieve a high level of centralized governmental power. After successive developments throughout the centuries, the rulers of the Ming Dynasty used the principle of interlocking governance for the demarcation of military and administrative districts. The boundaries between neighboring regional military commissions (dusi) and provincial administration commissions (buzhengsi) were closely interlocked. In addition there were also some interspersed military districts which were under the direct jurisdiction of the Five Chief Military Commissions of the central government. One classic example of this is the Tongguan Guard which was located on the border between the Henan and Shaanxi Provinces, and which had been established in the ninth year of the Hongwu Emperor. This article traces the development of the Tongguan Guard during the Ming and Qing Dynasty, and the accompanying changes in power structures at the local and central governmental level. At first, the Tongguan Guard was under the jurisdiction of the Henan dusi, but in the sixth year of the Yongle Emperor, it was put under the jurisdiction of the Beijing Defense Branch Rear Chief Military Commission. After this office was abolished, it was put under the jurisdiction of the Rear Chief Military Commission and finally under the direct jurisdiction of the Center Chief Military Commission. The jurisdiction of the Tongguan Guard changed repeatedly a phenomenon which was directly related to the fact that the jurisdiction of these regions was too distant from Beijing, creating enormous difficulties related to their management. These garrisons were originally placed under the direct jurisdiction of the Five Chief Military Commissions of the central government. This was done in an effort to assert central governmental power and curtail local power structures. However, as the ruling power of the central government grew weaker, the garrisons could no longer strengthen central governmental power but rather became dependant on local power bases for their normal operation. Analysis of the transformations in the power structures of these garrisons can thus be used to understand the rise and fall of centralized power in the Ming Dynasty. The system of interlocking districts of jurisdiction which served to check and balance power was also clearly manifest at the boundaries between Military Farms and civilian regions on the county and district level. The seventy-two military farms of the Ming Dynasty Tongguan Guard were dispersed between two districts and seven counties. In each of these districts and counties, interlocking power structures between military farms and civilian areas were manifest. Owing to the intermingling of military and civilians, and the superior strength of the army in relation to civilians, the misappropriation of local resources by the military became common-place. After the invasion of the Manchus, the Tongguan Guard was abolished and later replaced on the county level. The garrisons of the Tongguan Guard were initially retained intact in order to facilitate the dispensing of soldiers’ provisions, the stockpiling of grain, and the collection of taxes. However, in the long run, the problems associated with the dispersal of the military farms into two provinces and eleven scattered districts or counties were difficult to resolve. Finally, the county-level Tongguan Guard was relegated to the sub-prefectural level and its area of jurisdiction dramatically reduced. After a long period of historical development, the early Ming Dynasty system of interlocking jurisdictional regions which had been intended to check and balance power gradually began to disintegrate.