The walls of the church form approximately a square, the length of which in the interior, exclusive of the apse, is 241 feet, and the breadth 224.[238] The dome measures 100 feet across from the edge of the cornice, but above the cornice the vaulted space is 104 feet across; it is 179 feet from the floor to the vertex. The dome rises above the square area on four huge arches, with a large semidome to the east and another to the west, each of which embraces three smaller spans. The lateral openings which thus pierce the east and western semidomes are covered by _conchs_, but the middle opening in each case has a cylindrical vault, that to the east being prolonged into the eastern apse. At the corners of the central square of the nave rise four large piers, which are joined by arches to four buttress piers in the northern and southern walls behind them. The arched openings connect the three parts into which the aisles are divided by the piers. On either side of each of the central openings from the eastern and western hemicycles rise other piers, which are pierced by narrow arched passages, running from north to south. The piers, eight altogether, carry the whole vaulting of the nave, as well as a part of that of the side aisles. Between the middle division of each aisle and the nave are four large columns with five arches on the ground floor, and on the first floor six smaller columns with seven arches. Above again is a wall with windows, filling up the great arches on the north and south sides under the dome. Each exedra has, on the ground floor, two large columns with three arches, and, on the floor above, six small columns with seven arches. The vault of each division of the aisles is supported on four columns. Those next the east and west walls of the church, eight in all, are square, the others are round. The divisions of the galleries follow those of the aisles underneath. The four main piers however were pierced by additional arched openings [now filled up] between the galleries and the nave. The part over the narthex opens to the nave by three arches, on coupled columns. Above is the immense semicircular window which fills up the central barrel vault at the western end. All the openings towards the nave in the upper aisles have marble parapets. The vaulting of the lower aisles rests on forty round columns and eight square ones, and in the galleries on sixty round columns, not including the coupled columns at the west; this makes in all a hundred round columns. Possibly the eight square pillars in the aisles were employed, so that this number should not be exceeded. In the walls are numerous large windows, and the dome is pierced by forty just above the cornice; thus light streams into the church from every quarter. Much of the dome, including the central circle of mosaic at the crown, can be seen from the Royal Door.