OCR Text |
Show 428 MR. E. W. L. HOLT-STUDIES I N [May 1, fish'' like the Cod the movements of the eye are such as can easily be attributed to the action of the eye-muscles, in flat fish, and especially in the genus Pleuronectes, when the fish is at rest or on the look-out for food, the eyes are considerably elevated above the rest of the head. If the fish is frightened by placing some object near the eyes, the latter are suddenly withdrawn into their sockets, but quickly rise again as soon as the cause of terror is removed. By close observation it m a y be seen that the retraction of the eyes is accompanied by a simultaneous swelling immediately behind the lower eye, i. e. in the region occupied by the recessus orbitalis of that eye. The swelling disapppears with the subsequent elevation of the eye. If a fresh fish is taken, and the skin removed as in Plate X X X . fig. 5, it will be seen that pressure on the lower eye has the effect of filling the recessus, but that as soon as the pressure is relaxed the organ empties itself back into the orbital cavity and the eye rises again. The same connection between the elevation of the eye and the elasticity of the recessus can be demonstrated in the case of the upper orbit, by pressure of the upper eye in either a living or a moderately fresh specimen, and no doubt the voluntary retraction of the upper eye when the fish is frightened is accompanied by a swelling of the region of the recessus on the blind side. I have not seen this, having no vessel suitable for making the experiment. N o w the eye is an organ of considerable weight, and is furnished with no protractor muscle, and it is impossible that such considerable protraction as one actually observes in the eyes of flat fishes can be effected by the mere relaxation of the oblique and recti muscles. Further, if the recessus of the upper orbit becomes in any way ruptured, the fish is no longer capable of elevating the upper eye, though its fellow continues to be raised and lowered as before. W e have seen that the structure of the recessus is such as to impart the greatest possible amount of elasticity to that organ, and I think that beyond doubt its function is simply to protract the eye and to regulate its vertical movements. It acts, as it were, after the fashion of a " push-ball." Assuming the protracted condition of the eye to be the normal state, it is obvious that it could not be retracted, even partly, into the orbital cavity without displacing a corresponding amount of the fluid contents of the latter. In the absence of a special diverticulum this would involve a stretching of the undifferentiated membranous wall; and though this structure is to a certain extent elastic, it is obvious that the stretching could not take place unless the cavity were entirely or mainly surrounded by non-resistant bodies. This, however, as we have seen when examining the topographical anatomy of the recessus, is not the case. It therefore follows that to admit of the retraction of the eye a special diverticulum must exist for the reception of the orbital fluid, and this w e find in the recessus. The eye being retracted, w e have seen that there is no apparatus |