CETACEA. 



585 



Fig. 274. 



Section of tlie eye of a Whale. 



" The tunica conjunctiva (g, g, fig. 274), 

 where it is reflected from the eyelid to the eye- 

 ball, is perforated all round by small orifices 

 of the ducts of a circle of glandular bodies 

 lying behind it. 



" The lachrymal gland* is small, its use 

 being supplied by those above-mentioned ; and 

 the secretion from them all, I believe to be 

 a mucus similar to what is found in the Turtle 

 and Crocodile. There are neither puncta nor 

 lachrymal duct (ductus ad nasum), so that the 

 secretion, whatever it be, is washed off into 

 the water. 



" The muscles which open the eyelids are 

 very strong; they take their origin from the 

 head, round the optic nerve, which in some 

 requires their being very long, and are so 

 broad as almost to make one circular muscle 

 round the whole of the interior straight mus- 

 cles of the eye itself. They may be divided 

 into four ; a superior, an inferior, and one at 

 each angle ; as they pass outwards to the eye- 

 lids, they diverge and become broader, and are 

 inserted into the inside of the eyelids almost 

 equally all round. They may be termed the 

 dilatores of the eyelids ; and, before they 

 reach their inseition, give off the external 

 straight muscles, which are small, and inserted 

 into the sclerotic coat before the transverse axis 

 of the eye ; these may be named the elevator, 

 depressor, adductor, and abductor, and may 

 be dissected away from the others as distinct 

 muscles. Besides these four going from the 

 muscles of the eyelid to the eye itself, there are 

 two which are larger, and enclose the optic 

 nerve with the plexus. As these pass outwards 

 they become broad, may in some be divided 

 into four, and are inserted into the sclerotic 

 coat, almost all round the eye, rather behind 

 its transverse axis. 



" The two oblique muscles are very long ; 

 they pass through the muscles of the eyelids, 

 are continued on to the globe of the eye, 

 between the two sets of straight muscles, and 

 at their insertions are very broad : a circum- 



* This is analogous rather to the Harderian 

 gland, being situated at the inner or nasal side of 

 the eyeball. 

 VOL. i. 



stance which gives great variation to the motion 

 of the eye. 



" The sclerotic coat (a, a, Jig. 274) gives 

 shape to the eye, both externally and internally, 

 as in other animals; but the external shape and 

 that of the internal cavity are very dissimilar, 

 arising from the great difference in the thick- 

 ness of this coat in different parts. The external 

 figure is round, except that it is a little flat- 

 tened forwards ; but that of the cavity is far 

 otherwise, being made up of sections of 

 various circles, being a little lengthened from 

 the inner side to the outer, a transverse section 

 making a short ellipsis. 



" In the Piked Whale ( Balanoptera ros- 

 trata) the long axis is two inches and three 

 quarters, the short axis two inches and one- 

 eighth. 



" The posterior part of the cavity is a 

 tolerably regular curve, answering to the dif- 

 ference in the two axises ; but forwards, near 

 the cornea, the sclerotic coat turns quickly in, 

 to meet the cornea, which makes this part of 

 the cavity extremely flat, and renders the 

 distance between the anterior part of the scle- 

 rotic coat and the bottom of the eye not above 

 an inch and a quarter. 



" In the Piked Whale the sclerotic coat, at 

 its posterior part, is very thick : near the ex- 

 treme of the short axis it was half an inch, 

 and at the long axis one-eight of an inch thick. 

 In the Bottle-nose Whale ( Hyperoodon), the 

 extreme of the short axis was half an inch 

 thick, and the extremes of the long axis about 

 a quarter of an inch, or half the other. 



" The sclerotic coat becomes thinner as it 

 approaches to its union with the cornea, where 

 it is thin and soft. It is extremely firm in its 

 texture where thick, and from a transverse sec- 

 tion would seem to be composed of tendinous 

 fibres, intermixed with something like carti- 

 lage ; in this section four passages for vessels 

 remain open. This firmness of texture pre- 

 cludes all effect of the straight muscles on the 

 globe of the eye by altering its shape, and 

 adapting its focus to different distances of 

 objects, as has been supposed to be the case in 

 the human eye. 



" The cornea (6, Jig. 274) makes rather a 

 longer ellipsis than the ball of the eye ; the side 

 of which are not equally curved, the pp 

 being most considerably so. It is a segmen , 

 of a circle somewhat smaller than that of the 

 eyeball, is soft and very flaccid.* 



" The tunica choroides resembles that of the 

 quadruped ; and its inner surface is of a silver 

 hue, without any nigrum pigmentum. The 

 pigmentum nigrum only covers the ciliary 

 processes (c, c), and lines the inside of the iris. 

 The retina (e} appears to be nearly similar to 

 that of the quadruped. 



" The arteries going to the coats of the eye 

 form a plexus passing round the optic nerve, 

 resembling in its appearance that of the sper- 

 matic artery in the Bull and some other ani- 

 mals. 



* Its laminated texture is well displayed in the 

 Whale; Leeuwenhoek counted twemy-iwo layers. 



