ORGAN OF SIGHT IN FISHES. 335 



fibrous layer of the sclerotic, wlucli undergoes the same change 

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

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

 strTicture is well displayed in the cornea of the Orthafiorixnis,^ 

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

 ' conjunctive membrane' {o), continued from the perijjhery of the 

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

 lining membrane is reflected from the posterior surface of the 

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

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

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

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

 aid from au 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, o, fig. 219, li; 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' discovered the rich marginal 

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

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

 tion of ))lood-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 

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

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

 Professor Midler 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 ; 

 Silurvs, Pimdodus, Sijnodon, Cohifk, and all the Eel-tribe, have 

 neither jiseudo' branchire nor clioroid vaso-ganglia. 



The most remarkable exception in the structure of the eye in 

 the present class is jn-esented by the Anableps, the comea of 

 which is bisected by an O20aque horizontal line, and the iris per- 

 forated by tw(5 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 



' XX. vol. iii. p. 147, prep. no. 1665. ^ lb. prep. no. 1649. 



' Lxxvi. ■' XX. vol. iii. (1836); p. 145, prep. 1656; and Lxvii. 



