158 ZOOLOGY. 



lodged in the soft parts ; but little if any of its substance 

 occupying the bony orbit. In size it does not much 

 exceed the eye of an ox.* The nerve of vision is long, 

 but not thicker than a crow's quill. It is closely en- 

 veloped by a dense fibrous sheath, four inches in peri- 

 meter, and which terminates on the posterior surface 

 of the globe. That portion of the conjunctiva which 

 immediately surrounds the cornea, and corresponds to 

 the aperture of the lids, has an intense black hue. It is 

 possible, however, that this dark membrane may be one 

 distinct from the conjunctiva; as it can be detached 

 from the latter tunic, and then resembles a layer of 

 cuticle, with a black pigment deposited beneath it.f 



The structure of the eye-ball is chiefly adapted for 

 aquatic vision. The cornea is dense and flattened ; 

 the anterior chamber of the eye is small, and contains 

 but little fluid ; the iris is a very coarse membrane, of 

 the dull- brown colour so general, if not invariable, in 

 the whale tribe, with a narrow zone of a lighter hue sur- 

 rounding its outer margin ; the crystalline lens is cer- 

 tainly not larger than the same part of the human eye 

 its form is nearly a perfect sphere ; the vitreous humour 

 is relatively abundant ; the retina is spread with beauti- 

 fully-arborescent blood-vessels, and presents a circular 

 bright spot at the insertion of the optic nerve. Im- 

 mediately beneath the retina, we find a dense mem- 



* In the adult female, the size of the globe, at its greatest circum- 

 ference, is seven and a half inches, its longitudinal and vertical diameters 

 two inches and a half. The interior, or cavity, is one inch and a half in 

 each of the last named directions, whilst its depth does not exceed two- 

 thirds of an inch. The transparent cornea, at its transverse and broadest 

 diameter, measures one inch ; and at its vertical or narrowest, three- 

 quarters of an inch. 



t A similar, but slighter dark tint around the cornea is not uncommon 

 amongst the dark-skinned varieties of the human race. 



