NERVOUS SYSTEM OF FISHES. 205 



brown pigment here stains the soft integument or ' conjunctive 

 membrane' (o), continued from the periphery of the cornea. In the 

 preparation of the eye of the same fish (No. 1649.), 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' (o) : this is of the 

 class of bodies called vascular- or 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* dis- 

 covered the rich marginal plexuses of vessels, whose trunks ('stamme') 

 have their origin in this body, which he believed to consist also 

 of a convolution 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 'muscle;' but I found that the supposed "fibres con- 

 sisted, in reality, of minute, parallel, and closely disposed vessels," 

 both arteries and veins, f Professor Miiller has detected an unex- 

 pected relation of co-existence between the choroid vaso-ganglion and 

 the pseudo-branchia, to which the Sturgeon, Lepidosiren, and the Pla- 

 giostomes are amongst the few exce^Jtions having the pseudo-branchife, 

 but not the vaso-ganglia. The genera Silurus, Pimelodus, Synodon, 

 Cohitis, and all the Eel- tribe, have neither pseudo-branchiye nor choroid 

 vaso-ganglia. The most remarkable exceptional peculiarity 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 perforated by tw^o pupils. 



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

 a spheroid, flattened anteriorly, around which part the integuments 

 commonly form a circular fold, yielding to the movements of the 

 globe. In Orthagoriscus the circular palpebral fold is deeper, and 

 is provided with a sphincter : in most Scomberoid and Clupeoid Fishes 

 there is an anterior and a posterior vertical transparent fold or 

 eyelid. In the eye of the Galeus (Prep. 1762.), you may see a nictitat- 

 ing membrane superadded to a well-developed circular palpebral fold of 



* i.xxvi. t '-^^'i- ^'*'' "'• (1836); p. 145. prep. 1650". : and i.xvu. 



