The plan of the church, as I have said, is octagonal, surmounted by a dome octagonal without but circular within.  From one of these eight sides the sanctuary is thrust out, flanked on either side by a circular chapel with a rectangular presbytery.  Standing obliquely across one of the two angles of the octagon, directly opposite this sanctuary, stretched the narthex flanked by circular towers.  The great octagon is divided into two stories, each of which has three windows upon each of the eight sides, the octagonal dome being lighted by eight single windows.

[Illustration:  S. VITALE]

Within the great octagon formed by the walls is a smaller octagon formed by an arcade of mighty piers which upholds the cupola.  This arcade contains a double loggia which thus runs round the whole church with the exception of the presbytery, where it ends in lofty tribunes.  It is upheld between the piers by columns of precious marble having capitals of the most marvellous beauty.

The space within this inner octagon is covered with a pavement laid down in the sixteenth century, consisting of all sorts of fragments of mosaics and marbles which that century destroyed.  The upper loggia was of old the gyneceo, the place of the women.  Nothing I think left to us in the world is more sumptuous and gorgeous than this interior.  Everywhere are glittering mosaics, precious slabs of marble, priceless columns of beautiful marble.  And where the mosaics have been destroyed or left unfinished, as in the cupola and the body of the church, baroque artists have filled the place with their paintings, paintings which in their own style are matchless and which it is now foolishly proposed should be destroyed.[1]

[Footnote 1:  We know nothing of any mosaics other than those in the presbytery and the tribunes, it may be that the church was covered with mosaic or was painted by the Byzantine artists, and this as well where the marble slabs now cover the piers as elsewhere.  If so it must have been glorious indeed.  Nothing that we can do can restore this work to us, and we achieve nothing but destruction by destroying the work that is now there.]

In our examination of the church we turn first to the presbytery, which is entirely encrusted with most precious marbles and mosaics.  In the midst of it stands the altar consisting of slabs of semi-transparent alabaster, within which of old lights were set.  The marvellously lovely piece which serves for the altar stone itself is supported by four columns, and that piece which serves for frontal is carved with a great cross between two sheep.  This altar had long disappeared, but piece by piece it was recovered; the beautiful altar stone itself was found behind an altar in a chapel now destroyed in this church, and was re-erected as we see it in 1899.

[Illustration:  Colour Plate S. VITALE:  THE PRESBYTERY]

In the same chapel stood till then the beautiful low fretted screens that now are set across the apse behind the altar, where indeed they remained till 1700, according to Dr. Ricci.  The lower part of the apse and the piers of the presbytery have been covered with fine marbles, some of which are ancient, but the vault, the lunettes, and the walls are entirely encrusted with gorgeous mosaics.