For the maintenance of the parallelism between the axes of the two eyes, it is evidently necessary there should be a consentaneous action of the superior recti as well as of the in ferior red; ; it is also necessary that the internal rectus and the inferior oblique of one eye should act with the external rectus and the superior oblique of the other; but it is by no means evident why it is necessary, in order to effect this, that the external rectns and the superior oblique should each have a nerve specially provided for them. We must not suppose we are explaining the necessity for this arrange ment by asserting that " if in place of the sixth nerves, the external recti muscles had received each a branch of the third nerve, it would have been impossible to make one of these muscles act without the other," because, as we have seen, there is TIO such irresistible innate ten dency in all the corresponding branches of the third nerve to consentaneous action. Assurning that the use of the oblique muscles is such as we have mentioned, it is certainly curious to observe that when corresponding muscles of the two eyes are intended to act together, as the superior rectus of one eye with the snperior rectos of the other, and the same with the infe rior recti, both muscles are supplied by the third nerve, but the external rectus which acts cotisentaneously with the internal rectus of the opposite eye has a separate nerve, the sixth, and the superior oblique, which acts with the infe rior of the opposite eye, has the fourth nerve entirely devoted to it.
There is one other phenomenon to which we may briefly allude, namely, the adaptation of the eye to distances. This will be found fully dis cussed under the articleVistox ; but it deserves a passing notice in this place, since one hypo thesis by which an attempt has been made to explain it is that a change is effected in the form of the eye by the action of its external muscles, some writers attributing the influence in ques tion to the recti muscles, and others to the obliqui. It seems by no means improbable that the action of both sets of muscles at the same time might have the effect of increasing the antero-posterior diameter of the eye-ball, mid thereby of adapting it for vision at stnall distances. The following experiment seems to
favour the notion that there is some muscular action in the adaptation of the eye to vision at very small distances.
Place a printed page closer to the eyes than the natural focal distance, and merely look upon the letters without making any effort to read them ; they appear confused and indis tinct; then by a considerable voluntary effort which cannot be long sustained, and which is attended with some pain, we may so adjust the eyes that the letters appear perfectly distinct and legible. This subject is attended with great difficulties, indeed it is scarcely possible to determine the precise mode in which adap tation is effected ; the hypothesis we have now mentioned appmrs at least as probable as any other ; it is, however, open to objections. The state of adaptation of the eye is often entirely changed in a very short time by the local action of narcotics, which at the same time dilate the pupil, and this change is effected without any apparent influence over the voluntary contrac tions of the muscles. AIost observers state that the eye is rendered long-sighted (presbyopic), while others have experienced an opposite result. Muller has recorded the results of the application of belladonna to his own eye ; the general effect was to render the eye presbyopic, but the capability of adaptation was not de stroyed by it. When the solution of belladonna was applied to one eye both eyes were affected, but in different degrees, so that both eyes could not be adapted to distinct vision of the same object at the same monient. These effects Nvere attended with great dilatation of the pupil, and inasmuch as they were probably in some way dependent on this, they are opposed to the hypo thesis that adaptation is effected by any action of the external muscles.
( G. Johnson.)