FISHES. 337 



subject, it has six muscles, of which the four right ones (Nos. 1, 2, 

 3, 4, fig. iii), arise from the bottom of the orbit near the circumference 

 of the optic foramen, and two oblique, (a 6,) arising from the an- 

 terior wall of the orbit, and are inserted transversly, the one above, 

 the other below, into the globe. The superior oblique is destitute of 

 the pully which changes the direction it has in quadrupeds. The 

 muscle which forms the siphon in quadrupeds is likewise wanted. 



The fourth pair of nerves are inserted into the superior oblique, 

 and the sixth pair into the abductor ; the others receive their nerves 

 from the third pair precisely as in the vertebrated animals. 



The intervals between the orbit, the globe, the muscles, nerves, 

 and vessels are furnished with a loose cellular tissue, filled with a 

 gelatinous fluid, adapted to afford facility to the movements of the 

 eye. 



There are neither lacrymal gland, nor puncta lacrymalia in their 

 eyes, and in effect this apparatus is not necessary to animals whose 

 eyes are constantly moistened by the water in which they live. 



In the rays and squalus the globe of the eye is borne upon a move- 

 able cartilaginous pedicle, attached to the bottom of the orbit between 

 the origin of the two rectus muscles ; an arrangement which gives 

 strength to its movements. 



The anterior surface of the eye (<?, fig. VII) is generally level or 

 slightly convex, and the aqueous fluid is but little abundant ; the 

 remaining part of the surface of the globe is spheroidal, at least more 

 or less approaching to the figure of a sphere, but sometimes is rather 

 irregular. The rays have the upper portion flat so that the eye, in 

 this species, presents the general form of a quadrant of a sphere. 



The eye of the anableps is very remarkable, it has two cornea; 

 which are separated by an opaque line, and two pupils perforated in 

 the same iris, so that they appear double ; but it has only one vitreous 

 humour, one crystalline, and one retina.* 



The crystalline lens of fishes (c/. fig. VI, VII, VIII) is sensibly 

 spherical, very voluminous, and has a less space for the vitreous 

 humour than the eyes of animals which live in the air. Its consist- 

 ence is very great; its nucleus, which is very hard, remains trans- 

 parent even in spirits of wine, its external layers are numerous, and 

 are divided into fibres in the direction of the meridians of this small 

 globe : its capsule is soft ; it is attached in a hollow of the vitreous by 

 a circular line, produced by the membrane of the vitreous, which 

 surrounds it very nearly like the horizon of a geographic globe, (a) 



ment of all the parts. In fine, fig. viii. is the globe cut across, and showing the 

 crystalline ligament. 



1. If we take their direction with regard to the axis of the eye, or to its exterior 

 surface, they are frequently very oblique : this probably is what made Albers say that 

 the dorad (coryphoena) has two right muscles, and four oblique ; but I have followed 

 the nomenclature used in the human subject. 



* See Lace"pede, Mem. de l'lnstitute, sc. math, et phys., t. 1 1, 179S», p. 372. 



0^* (a) Dr. Brewster by some investigations lately undertaken, has shown that 

 the crystalline lenses in all animals are composed of fibres, and that if the lamina; 

 VOL. II. z 



