On so-called paresis of divergence / by George A. Berry.

  • Berry, George A. (George Andreas), 1853-
Date:
[1882]
Catalogue details

Licence: Public Domain Mark

Credit: On so-called paresis of divergence / by George A. Berry. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    as superfluous the assumption of the existence of a special divergence innervation, and consequently of a definite centre for this innervation.' I shall content myself by repeating here only two arguments which can be adduced against the existence of a divergence centre. It is evident that divergence beyond parallelism of the axes is never a physiological requirement, and therefore that the only imaginable use for a divergence innervation could be to cause the return of the eyes to a position of parallelism from that of convergence. Like convergence, divergence must be primarily regulated by fusion, and as fusion is not called for with diverging axes, no habit of absolute divergence can be naturally acquired. Yet it surely ought to be possible to induce further divergence in the interests of single vision by means of abducting prisms were there an active centre for divergence. In the case of convergence, one finds that very little practice is required to overcome adducting prisms of a strength which cause a very much greater degree of convergence than is ever called for physiologically. No doubt abducting prisms can also be overcome so that it is possible to have an absolute divergence. But there is a limit to the degree of this absolute divergence even after prolonged exercise with prisms. This limit coincides with the degree of divergence which is assumed when there is a complete loss of any converging power. In other words, the limit of possible divergence is the same as the starting-point of convergence. This is the physiological reason for disbelieving in a divergence centre. A more important, more convincing evidence is afforded by certain conditions of paralysis. For instance, cases occur in which there is a complete loss of power of