(a) Permanent 10 cases. V. more than 1/7 in 5; V. less than 1/36 in 3 cases, 2 excluded (one on account of large anterior synechia, one on account of choroiditis of the macula lutea).
(b) Periodic 9 cases. Among them one with anisometropia of more than 2 D. V. more than 1/7 5 cases. V. = 1/9 1 case; 3 cases excluded on account of complications.
F. Divergent squint in myopia more than 6·5 D.
(a) Permanent 8 cases. V. more than 1/7 4 cases, 4 excluded on account of choroiditis of the macula lutea.
(b) Periodic 10 cases. V. to 1/7 9 cases; V. = 1/12 in one case.
Table of Refraction and Visual Acuteness in Divergent Squint.
| Permanent. | V. to 1/7. | V. < 1/7 to V. = 1/12. | V. < 1/12 to V. = 1/36. | V. < 1/36. | Excluded. | Periodic. | V. to 1/7. | V. < 1/7 to V. = 1/12. | V. < 1/12 to V. = 1/36. | V. < 1/36. | Excluded. | |
| Hypermetropia | 4 | 1 | — | — | 1 | 2 | 5 | 3 | 1 | 1 | — | — |
| Emmetropia | 37 | 18 | — | 10 | 3 | 6 | 28 | 27 | 1 | — | — | — |
| Myopia to M. 2 D. | 24 | 15 | 2 | 3 | 2 | 2 | 23 | 23 | — | — | — | — |
| M. 2 D. to 4 D. | 17 | 9 | 1 | 2 | 1 | 4 | 8 | 7 | — | 1 | — | — |
| M. 4 D. to 6·5 D. | 10 | 5 | — | — | 3 | 2 | 9 | 5 | 1 | — | — | 3 |
| M. more than 6·5 D. | 8 | 4 | — | — | — | 4 | 10 | 9 | 1 | — | — | — |
| 100 | 52 | 3 | 15 | 10 | 20 | 83 | 74 | 4 | 2 | — | 3 |
It follows then from this, that periodic absolute divergent squint is just about as frequent as the permanent form and that both become more rare as the degrees of myopia increase. As, however, in spite of this, myopia is present in about 60 per cent. of all cases, the connection can be no other than this, that myopia frequently unites itself with insufficiency of the interni and preponderance of the externi; in this respect, as in every other, myopia and hypermetropia are directly opposed.
The setting up of a "hypermetropic divergent strabismus," dependent on hypermetropia, seems to me only to show how much people have been carried away by the idea that the cause of the squint must be given by the state of refraction. Isler claims 17 to 29 per cent. of the cases for hypermetropic divergent strabismus; of these, however, the half possess only slight hypermetropia of 2 D. or less, which perfectly agrees with the fact that the same observer has also found in convergent squint a remarkably high percentage of the lower degrees of hypermetropia.
Whether squint originates in the permanent or periodic form depends chiefly on whether the movement of convergence is retained or lost. There are cases of considerable divergent squint, in which the near point of the convergence is scarcely removed, while on the other hand, the physiological innervation for convergence may be lost, without absolute divergence ever being brought about. In a number of emmetropic or slightly myopic cases with absolute preponderance of the externi, the physiological connection between accommodation and convergence is maintained in a relaxed way; thus, for example, it is impossible to converge voluntarily to a large object, as, for instance, a pencil held in the vertical line, while accurate convergence immediately follows on reading at the same distance; in other cases accommodation can be exerted to the near point, without inducing the slightest impulse to convergence. This circumstance is worthy of consideration for the prognosis of the operation. A mere relaxing of the tie between accommodation and convergence may be strengthened by practice, but if the impulse to innervation is completely lost, it will scarcely be possible to restore it again; as after complete laying aside of absolute divergence the relative form still continues to exist.