ORGAN OF SIGHT IN FISHES. 335 



fibi*ous layer of the sclerotic, which undergoes the same change 

 of tissue, and forms the posterior layer of the cornea. This 

 transparent window of the eye-capsule is quite flat : its laminated 

 structure is well displayed in the cornea of the Orthagoriscus, 1 

 and a dark-brown pigment here stains the soft integument or 

 e conjunctive membrane' (0), continued from the periphery of the 

 cornea. In the eye of the same fish, 2 a very delicate layer or 

 lining membrane is reflected from the posterior surface of the 

 cornea, answering to the ' membrane of the aqueous humour ' of 

 land animals : this humour exists in very small quantity, just 

 enough to lubricate the iris in the eyes of Fishes : the medium 

 through which the rays of light reach the eye needs no refractive 

 aid from an aqueous fluid interposed before the lens in the globe 

 itself. 



Amongst the most characteristic peculiarities of the eye in 

 the typical or Osseous Fishes is the so-called ' choroid gland ' 

 fig. 216, 0, fig. 219, h ; this is of the class of bodies called ' vaso- 

 ganglions : ' it usually presents a dark red colour, and lies between 

 the ' silvery' and ' vascular ' layers of the choroid, more or less 

 encompassing, in the shape of a horse-shoe or bent magnet, the 

 entry of the optic nerve. Dr. Albers 3 discovered the rich marginal 

 plexuses of vessels, ' the roots of which have their origin in this 

 body,' and the body itself he believed to consist also of a convolu- 

 tion of blood-vessels. Ordinary dissection, however, shows its 

 compact substance to be arranged in parallel straight lines running 

 between the convex and concave borders, and it has been called a 

 e muscle ; ' but the supposed ( fibres consisted, in reality, of minute, 

 parallel, and closely-disposed vessels, both arteries and veins.' 4 

 Professor Muller has detected a relation of coexistence between 

 the choroid vaso-ganglion and the pseudo-branchia, to which the 

 Sturgeon, Lepidosiren, and the Plagiostomes are amongst the 

 exceptions, having the pseudo-branchias but not the vaso-ganglia ; 

 Silurus, Pimelodus, Synodon, Cobitis, and all the Eel-tribe, have 

 neither pseudo- branchiae nor choroid vaso-ganglia. 



The most remarkable exception in the structure of the eye in 

 the present class is presented by the Anableps, the cornea of 

 which is bisected by an opaque horizontal line, and the iris per- 

 forated by two pupils. 



The general form of the eyeball, or rather its capsule, in Fishes, 

 is a spheroid, flattened anteriorly, around which part the integu- 

 ments commonly form a circular fold, yielding to the movements 



1 xx. vol. iii. p. 147, prep. no. 1665. - lb. prep. no. 1649. 



xx. vol. iii. (1836); p. 145, prep. 1656; and lxvii. 



4 



