220 NATURAL HISTORY SOCIETY OF DUBLIN. 



sight results). The causes of accommodation have been assigned to al- 

 teration in the curvature of the cornea, to difference in the size of the 

 pupil, alteration in the curves and position of the lens, and to altera- 

 tion in the shape of the globe, by pressure of the external muscles. 

 Modern research proves that in the act of accommodation the lens is al- 

 tered in its anterior curve, and in its position ; and these changes are 

 attributed to the ciliary muscle, it being generally denied that the ex- 

 ternal muscles have any participation in the process. I have always 

 held that the external muscles of the eye acted not only as locomotory 

 agents, but that they also, to a certain extent, influenced the accommo- 

 dation ; and I think the eye now under consideration strengthens this 

 belief; for in it there is an arrangement whereby the anterior segment 

 of the globe may be approximated to the posterior. "When the animal 

 is in the water, and in search of its prey, I think its sight is perfect ; 

 and I believe the recti are then in powerful action, and retract the an- 

 terior segment, at the same time that they will, together with the ob- 

 liqui, compress the globe laterally. In this action the lens will be 

 pushed forward towards the cornea, and the cornea itself possibly flat- 

 tened. If the ciliary muscle act at the same time in increasing the 

 convexity of the lens, we have this animal's eye assuming the same 

 form and arrangement as the fish's eye. In these motions the posterior 

 segment will, I believe, remain stationary, the anterior moving in rela- 

 tion to it on the thin hinge-like part of sclerotic. 



Dr. Bennett observed, that the principal point dwelt upon by Mr. 

 "Wilson seemed to be the increased thickening, in the Seal, of the 

 capsule of the eye posteriorly and anteriorly, with a zone passing some- 

 where near the vertical meridian transversely round the eye, so as 

 to leave the greatest weakness of the wall along that zone. They had 

 at present in this question to deal with very imperfect data. He saw 

 the body of the Seal, but did not make a particular examination of its 

 eyes at the time ; but on the facts he had noticed, there appeared to be 

 a curious approximation between the eye of the Seal and those of the 

 other aquatic Mammalia — viz., the Cetacea — namely, an enormous 

 thickening in both of the posterior as well as the anterior parts of the 

 sclerotic. This thickening, in some individuals, amounted to so much 

 as two inches ; and there was a fine specimen of it in the Museum of 

 Trinity College. The fact bore especially upon the present question, as 

 Mr. Wilson's theory as to the power of ocular adjustment in the Seal was 

 different from that which was generally received. Mr. Wilson's idea 

 was, that the antero-posterior axis was elongated under a lateral pres- 

 sure of muscles external to the globe of the eye, that pressure taking 

 effect owing to the weakness of the coat along the middle zone, and the 

 anterior muscles forming a fixed point. His objection to this supposed 

 action of the muscles was, that it would have the effect of shorten- 

 ing the long axis of the eye. He did not believe that the external 

 muscles had anything to say to the focussing of the eye. If they 

 had, it would be by pressing the anterior half of the globe of the eye 



