Page:Observations on Man 1834.djvu/170

 attollens and deprimens; and if the eye be but a little open, the light cannot fall upon these last at all.

Secondly, If a luminous object be placed upon the right side, so as that the light shall fall in a particular manner upon the tendinous expansion of the abducens of the right eye, and of the adducens of the left, it may by contracting these muscles make the eyes move in a congruous manner, turning them towards itself; and the tendency of the eyes to move towards the right side ought not to cease, till the adducens and abducens in each eye have an equal quantity of light fall upon them from the luminous object, i.e. till the optic axis be directed to it. In which case the eyes would be in æquilibrio, as far as they are under the influence of the light which falls upon the tendinous expansions of the straight muscles.

Thirdly, It agrees remarkably with the two last paragraphs, that new-born children move their eyes in a congruous manner; that the motions are chiefly to the right and left, scarce upwards and downwards at all, the eyelids being seldom so much opened as to expose the tendinous expansions of the attollens and deprimens; and that their eyes are frequently turned towards luminous objects, such as a candle, or a window.

Fourthly, But it is not necessary, that the eyes of new-born children should always turn to the luminous object, or remain fixed upon it. For every muscle, when it has exhausted itself by contraction, gives place to its antagonist of course. If therefore the luminous object be much on one side, the eyes ought to turn back from it almost immediately. To which we may add, that various luminous objects generally affect the eyes at the same time; that the four straight muscles do naturally balance each other, and keep the eyes in a right forward position; and that the oblique muscles scarce favour any particular oblique position, though they do prepare the eye to turn with greater facility, in compliance with the contraction of any one of the four straight muscles. There are therefore sufficient sources for a variety of motions in the globes of the eyes, without destroying their congruity.

Fifthly, It is worthy of attention here, that the attollentes and deprimentes do not want the same external influence of light to make them move in a congruous manner, as the adducentes and abducentes; inasmuch as one adducens, and one abducens, must act together to make the eyes move congruously to the right and left; whereas the two attollentes, and two deprimentes, act together in the congruous motions upwards and downwards. As far therefore as the nerves of one side sympathize with the corresponding nerves of the other in the influences which descend from the brain, there will be a natural tendency in the eyes to move upwards and downwards in a congruous manner, and to the right and left in an incongruous one. And this suits well with the greater exposition of the tendinous expansions of the