adducentes and abducentes, before taken notice of. For what reason so many pairs of nerves are concerned in the motions of the globe of the eye, and of the eyelid, remains to be inquired.
Sixthly, It may perhaps be, that the light which passes in at the pupil has some efficacy in moving the globe of the eye, either by unknown communications in the brain between the optic nerve, and the third, fourth, and sixth pairs, or perhaps by penetrating in a small degree through the retina, choroides, and sclerotica, to the four straight muscles. If this last influence could be allowed, it would oppose that exerted upon the tendinous expansions; but would, however, join with it in preserving the congruity of the motions.
Seventhly, As the two oblique muscles neither have tendinous expansions exposed to the light, nor adhere to the globe of the eye, except just at their insertion, they cannot be under either of the influences here supposed to affect the straight muscles, but must be subject chiefly to those which descend from the brain; thus acting almost uniformly, unless in particular agitations of the whole nervous system. And this agrees well with the fact, and with the uses generally assigned to these muscles, viz. those of keeping the eye in a moderate suspension always, and drawing it out on eminent occasions.
Eighthly, The circumstances which occasion squinting in young children, agree well with the theory here proposed. Thus, if a child be laid so into his cradle, as that one eye shall be covered, the external influences of light cannot operate upon it. And if this be often repeated, especially while the association which confirms the congruity of the motions is weak, the eye which is covered will obey the influences which descend from the brain, and turn upwards and inwards for the most part. What turns the scale in favour of this position, remains to be inquired.
The second of the external motions is that of the eyelids, or the actions of the elevator, and orbicularis palpebrarum. What excites the first to constant action during the whole time that the new-born child is awake, is difficult to say. Perhaps the action of light upon the white of the eye, sending vibrations under the upper eyelid, and thence into the fibres of the elevator: or the direct action of light through the skin; for it is a cutaneous muscle: or influences which descend into the third pair from the second, i.e. the optic nerve: or the friction from the globe of the eye in its motions, which may also make the eyelid sympathize in motion with the eye: or the aggregate of all these. As to the orbicularis, it is evidently put into action by irritations affecting the eye, as from dust, flies, &c. even in adults.
The third of the external motions, or the contraction of the lacrymal and other neighbouring glands, arises from irritations in the eye, nose, fauces, and scalp of the head. The manner in which these irritations operate, has been sufficiently explained under Prop. LIII.
