§ XXVI. It will be part of my endeavor, in the following work, to mark the various modes in which the northern and southern architectures were developed from the Roman: here I must pause only to name the distinguishing characteristics of the great families. The Christian Roman and Byzantine work is round-arched, with single and well-proportioned shafts; capitals imitated from classical Roman; mouldings more or less so; and large surfaces of walls entirely covered with imagery, mosaic, and paintings, whether of scripture history or of sacred symbols. The Arab school is at first the same in its principal features, the Byzantine workmen being employed by the caliphs; but the Arab rapidly introduces characters half Persepolitan, half Egyptian, into the shafts and capitals: in his intense love of excitement he points the arch and writhes it into extravagant foliations; he banishes the animal imagery, and invents an ornamentation of his own (called Arabesque) to replace it: this not being adapted for covering large surfaces, he concentrates it on features of interest, and bars his surfaces with horizontal lines of color, the expression of the level of the Desert. He retains the dome, and adds the minaret. All is done with exquisite refinement. § XXVII. The changes effected by the Lombard are more curious still, for they are in the anatomy of the building, more than its decoration. The Lombard architecture represents, as I said, the whole of that of the northern barbaric nations. And this I believe was, at first, an imitation in wood of the Christian Roman churches or basilicas. Without staying to examine the whole structure of a basilica, the reader will easily understand thus much of it: that it had a nave and two aisles, the nave much higher than the aisles; that the nave was separated from the aisles by rows of shafts, which supported, above, large spaces of flat or dead wall, rising above the aisles, and forming the upper part of the nave, now called the clerestory, which had a gabled wooden roof. These high dead walls were, in Roman work, built of stone; but in the wooden work of the North, they must necessarily have been made of horizontal boards or timbers attached to uprights on the top of the nave pillars, which were themselves also of wood.[21] Now, these uprights were necessarily thicker than the rest of the timbers, and formed vertical square pilasters above the nave piers. As Christianity extended and civilisation increased, these wooden structures were changed into stone; but they were literally petrified, retaining the form which had been made necessary by their being of wood. The upright pilaster above the nave pier remains in the stone edifice, and is the first form of the great distinctive feature of Northern architecture--the vaulting shaft. In that form the Lombards brought it into Italy, in the seventh century, and it remains to this day in St. Ambrogio of Milan, and St. Michele of Pavia.