218 NATURAL HISTORY SOCIETY OF DUBLIN. 



been brought to light, many theories and explanations previously re- 

 ceived have been either refuted or confirmed, and, what is of special 

 moment to the Zoological Society, as well as interesting to the medical 

 practitioner, the cause of death is generally ascertained. 



The mortality of the Seals in the Zoological Gardens is much to be 

 regretted by the public, as they are amongst the most interesting of the 

 animals in the Collection. The last two appear to have died of starva- 

 tion, and I believe the disinclination to eat was caused as much by phy- 

 sical suffering, from injuries received in their capture, as from mental 

 suffering in consequence of their captivity. It is to be hoped the next 

 specimen may be obtained uninjured and young. 



The Seal is a carnivorous mammal, living partly in the water, and 

 partly on the land ; subsisting in adult life, I believe, entirely on fish, 

 molluscs, and other aquatic animal products. Besides the external con- 

 figuration and modifications suitable to the animal, and the special 

 contrivance whereby the venous blood is retarded in its course to the 

 lungs, and thns the number of respirations diminished, and the animal's 

 power of remaining submerged increased, one of the most interesting 

 peculiarities of the Seal is its capability of seeing in two media of such 

 different densities and refractive power as air and water. The organ of 

 vision, therefore, must possess some special mechanism, whereby this ac- 

 commodation takes place ; and as the subject of distantial adjustment is 

 one of very great importance, as well to the medical practitioner as to 

 the metaphysician ; and as the structure and form of the animal's eye 

 may be unknown to some of the Members of the Natural History So- 

 ciety, I have not hesitated in bringing the subject under your notice. 



The peculiar arrangement of the case of the eye, to which I wish 

 more especially to direct your attention, was discovered by Blumenbach. 

 The globe is large ; its antero-posterior diameter is the larger, and mea- 

 sures 1 1 of an inch, while the lateral is 1 T S F , or ~ less. Some time after 

 removal from the orbit, my attention was arrested by a manifest sulcus 

 at the transverse diameter of the globe, so that the ball appeared divided 

 into an anterior and posterior segment, tbe two having fallen together. 

 The sclerotic or protecting case is a dense, firm membrane, enclosing 

 the sensitive and mechanical apparatus ; externally it is smooth and 

 polished, and spherical in shape, in the recent eye. It consists of two 

 wedge-shaped portions, joined at their apices at the transverse diameter — 

 the anterior 1 inch in length ; its base or broadest part, ^ thick at the 

 junction of cornea, sclerotic, and iris, gradually lessens in thickness from 

 before backwards, until it becomes thin and semitransparent, allowing 

 the dark-coloured choroid to shine through. From this point it gradu- 

 ally increases in thickness from before backwards, until it reaches to 

 the optic nerve, near which it is ^ inch, the same as in front ; the 

 length of this portion is 1^ inch. The internal surface of the anterior 

 portion is rough, and appears composed of circular fibres ; the posterior, 

 on the contrary, is smooth and polished, and seems composed of longi- 

 tudinal fibres ; in both the thickening appears to take place on the in- 



