336 FISHES. 



above by the principal frontal, bounded in front and behind by the 

 anterior and posterior frontals, the frame of this orbit is completed 

 beneath by the chain of suborbitary bones; the anterior sphenoid, 

 with the membranes attached to it occupy the bottom ; lastly, its floor is 

 in part supported by the pterygoidean, and by a more or less con- 

 siderable portion of the other bones of the pterygopalatine appa- 

 ratus. 



The position, direction, and size of the eye in fishes, vary to 

 infinity. 



In some, they look towards the heavens, and are frequently very 

 close to each other ; in others, far asunder, placed at the sides, and 

 even directed somewhat downwards. But in all this variety of 

 direction, we observe the most extraordinary one in the genus of 

 pleuronectes (turbots, plaice, soles, &c,) in which the two eyes are 

 placed one above the other, or at the same side of the head. In certain 

 fishes of the genera, eels, and silures, the eyes are so small as to be 

 scarcely perceivable ; Avhilst in others, such as the priacanthus, or the 

 pomatomus, they exceed by the proportional diameter any thing with 

 which we are acquainted in the superior classes. It may, however, be 

 stated, that the eye is larger in fishes in general, and particularly that 

 the pupil is large and open, a disposition which is well adapted for 

 collecting the rays of light at the bottom of the Avater, where they 

 penetrate in such small quantity. 



It has no true eye-lashes : the skin passes over the eye, and forms 

 there a conjunction slightly adherent which is often sufficiently tran- 

 sparent to permit the rays of light to pass to that organ. In certain 

 fishes, for example the eel, it passes without causing the least fold : 

 there are some, as the cecilia and gastrobranchiae, in which it re- 

 mains opaque, and hides all traces of the eye. In others, such as the 

 mackarel, and the herring, it forms an adipose fold before and behind ; 

 but these folds are fixed, and without muscles or motion : in the 

 squalus they are slightly moveable at the inferior border of the orbit. 

 Sometimes, as in the moon-fish, the skin is puffed out around the eye, 

 and is furnished internally with fibres that constitute a kind of 

 sphincter, whose action is counterbalanced by several layers of fibres 

 in the direction of the rays. 



The globe of the eye* is slightly moveable. Like that of the human 



lachyrmal organs (Erlang 1803); in Muck's thesis on the ophthalmic ganglion 

 (Landshut 1815) ; and in M. Massalien's, on the eyes of the tunny, and of the 

 cuttle-fish (Berlin 1815). Lastly, M. Jurine has published in 1821, in the first volume 

 of the Physical Society of Geneva, observations on the eye of the tunny, in which lie 

 throws out important remarks on the eyes of other fishes. 



* Fig. ii. pi. vii. represents the globe of the eye, entire, seen from behind, the 

 muscles being separated, and exhibiting the hole for the nerves ; the oily body would 

 appear, through the selerotic. In fig. iv. the selerotic is opened, and the lobes 

 separated ; the optic nerve is denuded of its covering, and unfolded ; the greasy 

 body may be seen naked beneath the selerotic. In fig. v. this greasy body is en- 

 veloped, as well as the silvered layer which envelopes the choroid, and the red body 

 may be seen naked, and in the form of an horse shoe, placed between this layer and 

 the choroid. Fig. vi. is the globe deprived of its selerotic and cornea, and viewed 

 laterally, showing the protrusion of the cristallin outside the pupil. Fig. vii. is a 

 section ofthe globe of the eye in the vertical direction, and showing the mutual encase- 



