The presbytery is approached from the inner octagon of the church under a triumphal arch.  In the curve of this we see amid much decorative ornament fifteen circular discs containing the head of Our Lord, the twelve Apostles, S. Gervasius, and S. Protasius.  Beneath these are two monuments variously formed, Dr. Ricci tells us, in the sixteenth century.  The four columns which they contain originally supported the baldacchino over the high altar here; three of them are of verde antico.  Framed by these columns are two Roman reliefs from a frieze originally in the Temple of Neptune, other parts of which are in the Sala Lapidaria in the Arcivescovado here, in the Louvre, in the Uffizi, in the Castello of Milan, and in the Museo Archeologico at Venice.  They are indubitably of course the oldest things in the church.

Within this triumphal arch upon either side rise the tribunes in which the upper loggia of the church itself comes to an end.  These tribunes, which are exceedingly beautiful, consist of two triple arches, one above the other on either side, and the columns which support them, with their marvellous capitals, are I suppose among the most glorious left in Christendom.  The arches themselves and the lunettes upon either side are encrusted with mosaics.  In the lunette upon the right on either side an altar gorgeously draped, Abel offers to God the firstling of his flock and Melchizedek Bread and Wine.  Upon the face of the arch we see Moses tending the sheep of Jethro, Moses upon Mount Hebron, and Moses before the burning bush.  In the lunette upon the left we have the sacrifice of Abraham of his only son, and the visit of the three angels to Abraham and Sara.  Upon the face of the arch we see Jeremiah the Prophet and Moses upon Mount Sinai.  Above, upon the balustrades, as it were, of the upper loggia we see angels upholding a circle in which is the sign of the Cross, and above again upon the face of the arches on either side the four Evangelists and their symbols.  The vault is entirely covered with ornaments in mosaic, amid which three angels rise and support with uplifted hands the central disc in which is represented the Agnus Dei.

Though these mosaics have suffered much from unforeseen disaster and from restoration they still delight us with their richness and splendour, and nothing I think can well be finer than their effect, their decorative effect as a whole.  They seem to hang there like some gorgeous Eastern tapestry of Persian stuff, as Dr. Ricci says, some unfading and indestructible tapestry of the Orient left by chance or forgetfulness in the old capital of the West.

We now turn to the apse, which we enter under a second triumphal arch upon the face of which we see upon the left the city of Hierusalem and upon the left Bethlehem.  A cypress stands at the gate of each, and between them two angels in flight uphold a discus or aureole having within it eight rays.  Above this again are three windows about which is spread a gorgeous decoration in mosaic.