454 Dynamic Theory. 



peculiarity in the optic nerve of fishes. This nerve instead of being- 

 cylindrical is composed of a number of folds side by side. It is shown 

 spread out in fig. 190. This will be mentioned again. 



FIG. 190. Eye of Sivord Fish. ( Owen.) 

 1. Outside Skin Modified over the Eye into 

 2. The Cornea. 



4. Iris and ciliary ligament. 

 5. Lens. 



6. Ossified Capsules of Sclerotic. 

 7. Retina, 



8. Uvea or Pigment Membrane. 

 9. Vascular Membrane. 

 10. Argentine Membrane. 



8, 9. 10. Together are called the Choroid Tunic, 

 11. Adipose and Cellular filling. 

 12. -Choroid Gland. 

 13. Optic Nerve spread out. 

 14. Ciliary Process border of the Uvea. 

 15. Falciform process of the Vascular Mem- 

 brane of the Choroid, pushed up and attached to 

 __ w the Lens. 



FIG. 190. 

 A good deal has been said to show the loss of function by disuse. 



The eye and other features of the organism that are built up lay habits 

 originating in the energ} T of light, are likewise lost or reduced by disuse 

 and darkness. Some examples have been given. The Amblyopsis 

 Speleus is a fish living in Mammoth Cave. It is entirely blind and is 

 destitute of even the orbital cavity. There is also a craw-fish in the 

 same cave, viz. , the Astacus pellucidus, whose eyes are likewise reduced 

 to rudiments, the pedicels or tubes remaining, while the facets have 

 disappeared. 



The Proteus of the subterranean lakes of Austria is a blind batrachian 

 whose connections above ground possess organs of sight. The eye of 

 the Proteus is reduced to the retina and pigment layer, the crystalline 

 lens and vitreous humor having disappeared. It is deeply seated in 

 the head and entirely covered with skin. The eye of the Mole is also 

 covered with skin. It Is very small. It has a simple retina and an 

 undeveloped lens, but one or both optic nerves are always aborted in the 

 adults. The embryo, however, always have both optic nerves well 

 developed. This proves the eye of the adult mole to be a rudiment in 

 process of atrophy, like the human inter-maxillary bone, or teeth in 

 the embryo whale, and it proves that the ancestors of the Mole were 

 possessed of good eyes, and were therefore of different habits. The 

 Mole inherits good eyes from his ancestors, but his environment is grad- 

 ually undoing what the enviroment of his ancestors gradually did. 



A peculiar famity of cra]js, the Pinothendae, in their second stage of 

 existence inhabit the interior of some mollusk. In their first or larval 

 state they are lively creatures with tail, limbs and well developed eyes, 

 and swim in the water. In order to develop into their second, mature 

 stage they manage to gain ingress to some of the branchial cavities of 



