In passing from a position of convergence to a position of parallelism, the lines of sight separate or diverge. This movement of divergence is a simultaneous, equal contraction of both externi; or, probably, of both actions combined. The eyes may even diverge somewhat beyond parallelism, as in overcoming prisms, base in, when looking at a distant object.
Vertical Divergence
The amount by which the lines of sight can separate in a vertical direction is very limited—at most but one or two degrees.
Orthophoria
The term orthophoria is used to denote an absolutely normal balance of the extrinsic muscles, just as the term emmetropia denotes a normal refractive condition. They are equally rare.
Heterophoria
The term heterophoria includes all those conditions in which there is a tendency to depart from normal balance, but which nature is able to compensate for; while the term also includes the conditions in which nature has been unequal to the task and an actual turning or squint has occurred.
Subdivisions
The subdivisions of these terms at first reading appear complicated, but prove simple enough on closer study, indicating only the direction of the turning or tendency to turn. For instance:
- Esophoria signifies inward tendency.
- Exophoria signifies outward tendency.
- Hyperphoria signifies upward tendency.
- Hypophoria signifies downward tendency.
- Cyclophoria signifies tendency to torsion.
- Esotropia signifies inward turning.
- Exotropia signifies outward turning.
- Hypertropia signifies upward turning.
- Hypotropia signifies downward turning.
- Cyclotropia signifies actual torsion.