§330. Plan of the Circus.--All of the Roman circuses known to us had the same general arrangement, which will be readily understood from the plan of the Circus of Maxentius shown in Fig. 139. The long and comparatively narrow stretch of ground which formed the race course proper (_arēna_) is almost surrounded by the tiers of seats, running in two long parallel lines uniting in a semicircle at one end. In the middle of this semicircle is a gate, marked _F_ in the plan, by which the victor left the circus when the race was over. It was called, therefore, the _porta triumphālis_. Opposite this gate at the other end of the arena was the station for the chariots (_AA_ in the plan), called _carcerēs_, "barriers," flanked by two towers at the corners (_II_), and divided into two equal sections by another gate (_B_), called the _porta pompae_, by which processions entered the circus. There are also gates (_HH_) between the towers and the seats. The exterior appearance of the towers and barriers, called together the _oppidum_, is shown in Fig. 140. [Illustration: FIGURE 140. OPPIDUM OF A CIRCUS] §331. The arena is divided for about two-thirds its length by a fence or wall (_MM_), called the _spīna_, "backbone." At the end of this were fixed pillars (_LL_), called _mētae_, marking the inner line of the course. Once around the _spīna_ was a lap (_spatium_, _curriculum_), and the fixed number of laps, usually seven to a race, was called a _missus_. The last lap, however, had but one turn, that at the _mēta prīma_, the one nearest the _porta triumphālis_, the finish being a straightaway dash to the _calx_. This was a chalk line drawn on the arena far enough away from the second _mēta_ to keep it from being obliterated by the hoofs of the horses as they made the turn, and far enough also from the _carcerēs_ to enable the driver to stop his team before dashing into them. The dotted line (_DN_) is the supposed location of the _calx_. It will be noticed that the important things about the developed circus are the _arēna_, _carcerēs_, _spīna_, _mētae_, and the seats, all of which will be more particularly described.