FISHES 



303 



running between the cornea and the sclera, while in Beryx the entire 

 muscle seems to pass from the cornea to the sclera without a choroidal 

 attachment (Rochon-Duvigneaud, 1943). 



The tensor choroidese is generally accej^ted as the precursor of the ciliary 

 muscle (of Briicke) of Sauropsida and Mammals, but in Fishes its function is 

 not clear ; it has been said to brace the retina and choroid when the lens presses 

 backwards ujjon the vitreous during accommodation (Beer, 1894), while the main 

 role in teleostean accommodation was ascribed to it by Bourguignon and 

 Verrier (1930). i 



The iris is complex in structure and frequently brilliantly coloured. 

 The continuation of the choroidal argentea over its anterior surface 

 gives it a metallic sheen and in addition bright pigments abound — 

 gold, scarlet, yellow, mauve and others, sometimes so dense that the 

 structure of the tissue or the arrangement of its vessels is completely 

 obscured (Plate II). In some species Beer (1894) foimd that a slow 

 change in colour could be induced by electrical stimulation, presumably 

 owing to contraction of the clu-omatophores ; a similar change has 

 been induced in the carp, Cyj^ririus, by the injection of adrenalin or 

 ablation of the hypoiDh3%sis (Rochon-Duvigneaud. 1943). The two 

 ectodermal layers conform to the usual pattern, the posterior being 

 non-pigmented almost half-way towards the pujDil, the anterior heavily 

 pigmented tlu'oughout its extent. From the latter are developed the 

 myoepithelial fibres of the sphincter muscle ; in most species a few 

 radial cells represent the elements of a dilatator muscle although in 

 some these may be marked (the sword-fish, Xijihias — Barraquer- 

 Cerero, 1952). Anterior to the ectodermal layer the heavily pigmented 

 vascular layer forms the forward continuation of the choroid, covered 

 superficially by the thick argentea ; while over a varying j^ortion of 

 the peripheral area of the anterior siuface of the iris the cellular 

 annular ligament spreads itself, filling up the angle of the anterior 

 chamber in contininty with its corneal extension. The iris is usuallj^ 

 supplied by two anterior ciliary arteries which enter in the horizontal 

 meridian on either side and run on the superficial surface straight 

 towards the pupil ; here they divide to form a circular arterial anasto- 

 mosis around the pupillary margin (Plate II, Fig. 5). The venous 

 drainage is by deeper vessels running beneath the argentea, and there- 

 fore hidden from view ; they are continuous with the choroidal veins 

 (J. Miiller, 1840 ; Virchow, 1882 ; Mann, 1929-31). 



The pujril is round or horizontally oval or pear-shaped, but in 

 general, even in the rare tyjjes wherein the sphincter forms a massive 

 band, the pupils of Teleosts are essentially immobile, the iris being 

 widely fixed to the posterior surface of the cornea by the aimular 

 ligament. As wath selachian irides. the pupils contract sluggishly and 



1 p. 646. 



Beryx 



Xiphias 



