vaulted, for a central dome would seem to furnish their very _raison
d'etre_. The central space was sometimes surrounded by a very thick wall,
in which deep recesses, to the interior, were formed, as at the noble
church of St George, Salonica (5th century?), or by a vaulted aisle, as at
Sta Costanza, Rome (4th century); or annexes were thrown out from the
central space in such a way as to form a cross, in which these additions
helped to counterpoise the central vault, as at the mausoleum of Galla
Placidia, Ravenna (5th century). The most famous church of this type was
that of the Holy Apostles, Constantinople. Vaults appear to have been early
applied to the basilican type of plan; for instance, at St Irene,
Constantinople (6th century), the long body of the church is covered by two
domes.

At St Sergius, Constantinople, and San Vitale, Ravenna, churches of the
central type, the space under the dome was enlarged by having apsidal
additions made to the octagon. Finally, at St Sophia (6th century) a
combination was made which is perhaps the most remarkable piece of planning
ever contrived. A central space of 100 ft. square is increased to 200 ft.
in length by adding two hemicycles to it to the east and the west; these
are again extended by pushing out three minor apses eastward, and two
others, one on either side of a straight extension, to the west. This
unbroken area, about 260 ft. long, the larger part of which is over 100 ft.
wide, is entirely covered by a system of domical surfaces. Above the conchs
of the small apses rise the two great semi-domes which cover the
hemicycles, and between these bursts out the vast dome over the central
square. On the two sides, to the north and south of the dome, it is
supported by vaulted aisles in two storeys which bring the exterior form to
a general square. At the Holy Apostles (6th century) five domes were
applied to a cruciform plan, that in the midst being the highest. After the
6th century there were no churches built which in any way competed in scale
with these great works of Justinian, and the plans more or less tended to
approximate to one type. The central area covered by the dome was included
in a considerably larger square, of which the four divisions, to the east,
west, north and south, were carried up higher in the vaulting and roof
system than the four corners, forming in this way a sort of nave [v.04
p.0908] and transepts. Sometimes the central space was square, somet