476 SPECIAL PHYSIOLOGY. 



In the non- Vertebrate Annulosa, e. g., the eyes are convex, and images of 

 surrounding objects, and also of the limbs and other appendages of the animal, 

 are received in a true, and not in a reversed as well as an inverted position : 

 those on the right-hand side of the animal, being received on the right-hand 

 side of each eye, and those on the left, on the left-hand side, without inver- 

 sion ; in such animals, the motor apparatus is governed on its own side. In 

 the Vertebrata, the right- and left-hand objects, and, therefore, the right and 

 left limbs, are seen on the opposite sides of the eye ; the guiding impressions 

 thus perceived, pass over through the opposite side of the sensorium, and from 

 thence, the motor stimulus again crosses the middle line in governing the 

 movements of those limbs. In this way, the reversion of the visual impres- 

 sions, necessitated by the structure of the eye, meets with a corrective action 

 in the government of the limbs. 



In the eyes of fishes, the sclerotic is very firm, and generally contains two 

 cartilaginous plates ; when, however, those are absent, the posterior fibrous 

 part is of great thickness ; in some fishes, indeed, the posterior part of the 

 coat is one inch and a half thick ; in certain large fishes, it forms even a bony- 

 cup, into which the cornea is fitted in front, and through an opening in which 

 the optic nerve enters behind. By this great strength of the sclerotic, which 

 reminds one of the similar provision in the whales, the sphericity of the pos- 

 terior part of the eyeball is maintained. Sometimes a cartilaginous or tendi- 

 nous pedicle connects the sclerotic with the bottom of the orbital cavity ; in 

 the skate, a distinct arthroidal articulation exists between the pedicle and the 

 eyeball. The choroid coat is composed of several layers ; the pigment, in the 

 osseous fishes, has a silvery hue in its superficial fibrous layer, but is black or 

 purple in its deeper layer ; between these two layers, is a red horseshoe-shaped 

 vascular organ, composed of tortuous bloodvessels, chiefly venous, and known 

 as the choroid gland, the use of which is yet unexplained. Amongst the car- 

 tilaginous fishes, in the sharks and rays, the outer layer of the choroid pig- 

 ment is of a dark color, and the deeper one has a metallic lustre. In the stur- 

 geon, and also in some osseous fishes, there exists a fold of the choroid, 

 forming the falciform process, on which is often found a branch of a ciliary 

 nerve, expanded at its extremity, and forming the campanula (Haller.) The 

 falciform process, like the pecten of birds, projects into the vitreous humor, 

 and is fixed to the back part of the capsule of the lens. In the conger-eel, 

 there are two such processes, by which the lens is suspended at two points. 

 The iris, for the most part, presents a bright metallic lustre, and has a large, 

 round, and slightly changeable pupil ; the pupil has, indeed, been described as 

 being quite motionless. The anableps is said to have a double pupil in each 

 eye ; in the rays, a broad black velum is found in front of the pupil. After 

 death, the form of the pupil in the fish's eye is often very irregular. The 

 retina usually reaches forwards to the border of the iris. It has been sug- 

 gested, that fishes, living at a very great depth in lakes, or in the ocean, must 

 be almost in a state of complete darkness ; and that either their retinae, with 

 the plica3 upon the falciform process, must be endowed with an increased sen- 

 sibility to light, or that they possess in their barbules, or other appendages, 

 sensory organs of touch which compensate them for their inability to see. 



In those Vertebrata which live in the air, the refracting powers of the eye 

 depend largely on the cornea and aqueous humor combined ; but in fishes, the 

 action of these parts must be comparatively slight, as their refractive power is 

 so little in excess of water, the medium from which the light immediately 

 enters the eye. The cornea, indeed, in fishes, is very flat, and the aqueous 

 humor is very much reduced in quantity, conditions the very opposite of those 

 found, for example, in the far-seeing birds of prey ; but the formation of distinct 

 images, by bringing the luminous rays to an exact focus on the retina, is accom- 

 plished chiefly by the crystalline lens, which is accordingly of very large size, 

 relatively to that of the entire eyeball, completely spherical, and unusually 

 dense, its internal laminae being almost as hard as horn. These characters 

 impart high refractive power to the lens in fishes ; its focus is accordingly very 

 short, and hence the vitreous humor, like the aqueous, is scanty, and its form 

 flattened, so that the lens is closely approximated to the retina ; in front, too, 

 the lens usually projects through the pupil into the anterior chamber of the 



