III. ORATORIES AND CHURCHES BUILT OVER THE TOMBS OF MARTYRS AND CONFESSORS. The sacred buildings of this class are, or were formerly, outside the walls, as burial was not allowed within city limits. To explain their origin and to understand their significance we must bear in mind the following rules. The action of the Roman law towards the Christians, that is, towards persons accused of atheism and rebellion against the Empire, resulted in the execution of those who were convicted. Except in extraordinary cases, the body of the victim could be claimed by relatives and friends and buried with due honors. In chapters vi. and vii. instances will be quoted of the erection of imposing tombs to the memory of Roman patricians, generals and magistrates, who were put to death under the imperial régime. The same privileges of burial were granted to the Christians, who preferred, however, the modesty and safety of a grave in the heart of the catacombs to the pompous luxury of a mausoleum above ground. The grave of a martyr was an object of consideration, and was often visited by pilgrims, who adorned it with wreaths and lights on the anniversary of his execution. After the end of the persecutions the first thought of the victorious church was to honor the memory of those who had fought so gallantly for the common cause, and who at the sacrifice of their lives had hastened the advent of the days of freedom and peace. No better altar than those graves could be chosen for the celebration of divine service; but they were sunk deep in the ground, and the cubicula of the catacombs were hardly capable of containing the officiating clergy, much less the multitudes of the faithful. Touching the graves, removing them to a more suitable place, was out of the question; in the eyes of the early Christians no more impious sacrilege could be perpetrated. There was but one way left to deal with the difficulty; that of cutting away the rock over and around the grave, and thereby gaining such space as was deemed sufficient for the erection of a basilica. The excavation was done in conformity with two rules,--that the tomb of the martyr should occupy the place of honor in the middle of the apse, and that the body of the church should be to the _east_ of the tomb, except in cases of "force majeure," as when a river, a public road, or some other such obstacle made it necessary to vary this principle. Such is the origin of the greatest sanctuaries of Christian Rome. The churches of S. Peter on the Via Cornelia, S. Paul on the Via Ostiensis, S. Sebastian on the Via Appia, S. Petronilla on the Via Ardeatina, S. Valentine on the Via Flaminia, S. Hermes on the Via Salaria, S. Agnes on the Via Nomentana, S. Lorenzo on the Via Tiburtina, and fifty other historical structures, owe their existence to the humble grave which no human hand was allowed to transfer to a more suitable and healthy place.