428 MR. E. W. L. HOLT — STTJDIES Df [May 1, 



fish'' like the Cod the movements of the eye are such as can 

 easily be attributed to the action of the e^ye-muscles, in flat fish, 

 and especially in the genus Pleiironectes, 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 some object near the eyes, the latter are suddenly withdrawn 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 may be seen that the 

 retraction of tbe eyes is accompanied by a simidtaneous swelling 

 immediately behind the lower eye, i. e. in the region occupied by 

 the recessus orbiialis 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 XXX. fig. 5, it will be seen that pres- 

 sure on the lower eye has the effect of filling the recessus, but that 

 as soon as the pressui'e 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 fi'esh specimen, 

 and no doubt the voluntary retraction of the upper eye w"hen the 

 fish is frightened is accompanied by a swelliug of the region of 

 the recessus on the blind side. I have not seen this, having no 

 vessel suitable for making the experiment. 



Now the eye is an organ of considerable weight, and is furnished 

 with no protractor muscle, and it is impossible that such consider- 

 able protraction as one actually observes in the eyes of flat fishes 

 can be efi'ected 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. 



We 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 follo^^■s that to admit of the retraction of the eye a 

 special diverticulum must exist for the reception of the orbital fluid, 

 and this we find in the recessus. 



The eye being retracted, we have seen that there is no apparatus 



