In rare cases of vertical heterophoria, each eye has either an upward tendency (anophoria) or a downward tendency (cataphoria). These cases must not be confused with anatropia and catatropia. In anaphoria and cataphoria, there is binocular fixation when both eyes are uncovered, while in anatropia and catatropia _one_ of the eyes squints. This shows the following squint condition: SQUINT LATERAL SQUINT The deviating eye turns in, or toward the nose: Esotropia (Strabismus convergens—Convergent Squint) The deviating eye turns out or toward the temple: Exotropia (Strabismus divergens—Divergent Squint) VERTICAL SQUINT The deviating eye turns up: Hypertropia (Strabismus sursumvergens) (Right or left) The deviating eye turns down: Hypotropia (Strabismus deorsumvergens) (Right or left) In addition to these lateral and vertical deviations, conditions exist in which the vertical meridian of one eye, instead of maintaining its parallelism with the vertical meridian of the other, either forms (or tends to form) an angle with it (cyclotropia), but is kept in position through muscular effort (cyclophoria.) Cyclotropia is usually due to paralysis of one of the ocular muscles, causing the vertical meridian of the affected eye to be tilted out or toward the temple (extorsion) or in toward the nose (intorsion). A tilting of the vertical meridian toward the right is also called dextrotorsion (or positive declination); and to the left, levotorsion or negative declination. 2. _Constant, Intermittent and Periodic Deviations_: A deviation, whether squint or heterophoria, may be present at all times (constant), or occasionally present and occasionally absent (intermittent). In this case we may have heterophoria alternating with orthophoria, or heterophoria alternating with squint; or squint alternating with orthophoria. We also find variations such as a squint for near and a heterophoria or orthophoria for distance; or a heterophoria for near and orthophoria for distance; or a constant squint for near and an intermittent squint for distance, etc. Again, a deviation may be periodic, in that its amount for distance may greatly exceed that for near, or vice versa. Opposed to a periodic deviation is one which is present, and in about equal amount, both for distance and near. Such a deviation, whether squint or heterophoria, is called “continuous.” 3. _Alternating and Uniocular Squint_: An alternating squint is one in which when both eyes are uncovered, so that both have a chance to “fix”; sometimes the right eye will deviate, sometimes the left. In uniocular (less properly monocular) squint, under the same conditions, one eye, either the right or the left, always “fixes” and the other always deviates. A uniocular squint is denoted as right or left, according to whether it is the right or left eye which deviates.