Oipnoans and other fishes A rudimentary accessory ol- 

 factory bulb is seen in the dipnoan Protopterus, but is appar- 

 ently lacking in the shark or actinopterygian. A lateral 

 diverticulum from the nasal sac has been observed in Lepi- 

 dosiren and Protopterus which resembles the structure of the 

 urodele. This diverticulum lies dorsolaterally. In Protopterus 

 there is a medial diverticulum which appears early and is 

 innervated by a distinct division of the olfactory nerve. 

 Later both lateral and medial walls have become folded. 

 The olfactory nerve has a bundle of fibers which might re- 

 present the vomeronasal nerve. 



It has been suggested that the ventromedial recess of the 

 nasal cavity oi Eusthenopteron, and the osteolepiform crossop- 

 terygians in general, corresponds to the Jacobson's organ 

 of the frog. Such a recess is lacking in the porolepiform 

 crossopterygians which have been assumed to be forerun- 

 ners of the urodeles. 



The nasal sac of Salmo arises from two parts, the medial 

 area of which may correspond to the organ of Jacobson. In 

 the fishes the olfactory epithelium lacks Bowman's glands 

 but has mucous cells. 



Taste buds 



The sensory cells of the taste buds are similar to those of 

 olfaction. Taste buds occur in fishes not only in the mouth 

 but on the lips and even over the surface of the head. In 

 some of the teleost fishes, taste buds occur over much of 

 the anterior part of the body (Ameiurus). These organs do 

 not show any particularly useful feature for comparative 

 study. They present one interesting facet, that of entodermal 

 origin in the pharynx and ectodermal origin outside of the 

 mouth. 



The eye 



The eye of the vertebrate shows a basic plan in its struc- 

 ture and in its development. The main variations are re- 

 lated to method of accommodation or focusing. 

 The maixMrxai eye In its development the eye begins as an 

 outgrowth of the brain wall. This invagination forms an 

 optic vesicle and then a cup which is notched ventrally as 

 a chorioid fissure. The cup induces the formation of an ecto- 

 dermal lens placode which invaginates and cuts off from the 

 overlying ectoderm. The development of the optic cup in- 

 duces the condensation of mesenchyme around it to form the 

 outer coats. Mesenchyme enters its inner cavity to give rise 

 to the vitreous body. 



In the fully formed eye the lens is a solid, relatively trans- 

 parent structure composed of fibrous cells; their long axis is 

 roughly parallel to the line of light transmission. The lens is 

 suspended by ligaments from a ciliary apparatus formed 

 mainly of mesodermal tissue. The iris is formed from the 

 margin of the optic cup which extends in front of the lens. 



Mesoderm is attached to its outer surface and gives rise to 

 the smooth radial and circular muscle fibers. 



A fluid chamber forms in front of the lens, and the iris 

 extends into this. The vitreous body, which has a few cells 

 associated with the fibrous material, fills the eye behind the 

 lens. This posterior chamber is lined behind by the retina, 

 the inner layer of the optic cup. Outside the retina is the 

 chorioid coat formed from the outer layer of the cup. The 

 whole eye is enclosed by sclera derived from the mesen- 

 chyme or mesoderm surrounding the cup. 



The optic nerve is formed from the fibers of the inner 

 ganglion cells of the retina; these fibers converge toward the 

 area of the chorioid fissure and follow the underside of the 

 optic stalk to the brain. 



Early in the development of the eye there is a hyaloid 

 artery supplying the vascular tunic of the lens. The hyaloid 

 artery passes through the chorioid fissure along with the 

 optic nerve. Later it sends branches to the retina, becoming 

 the central retinal artery; the vessel to the lens disappears 

 leaving the hyaloid canal through the vitreous body. The 

 chorioid fissure closes except for the area of penetration of 

 the nerve and artery. Later the optic stalk disintegrates 

 forming glial cells associated with the optic nerve. 



Mechanisms of accommodation vary. The ciliary body of 

 most mammals has circular as well as radially arranged 

 muscle fibers. Accommodation may involve contraction of 

 the ring and radial muscles by pulling in the direction of the 

 lens, thus reducing the tension in the suspensory ligaments 

 and allowing the lens to round up through its own elasticity. 

 Thickening of the lens focuses near objects. In the horse, 

 movement of the head or the eye in its socket shifts the 

 image to different parts of the retina, which, because of the 

 shape of the eyeball, are nearer or farther from the lens. 



The eye of ofher veriebraies Within the vertebrates there 

 are variations in eye structure but not eye origin. In the myxi- 

 nid the eye is degenerate and buried beneath several layers 

 of muscle. The eye consists of little more than a retina en- 

 closed by some chorioid cells, and there is no lens. The optic 

 nerve is very small. The muscles of the eye are lacking as 

 are their nerves. 



In the lamprey, the eye is much the same as in the other 

 vertebrates; it lies in a socket formed of blood and lymph 

 spaces and has well-developed muscles. The eye is firmly at- 

 tached to the rim of the orbit but not to the overlying skin, 

 the conjunctiva. Focusing is effected by a body-wall muscle, 

 lying behind the eye, which inserts on the hind margin of 

 the cornea. The pull of this muscle flattens the normal 

 curvature of the cornea and pushes the lens inward. 



In the fishes there are other devices for accommodation 

 which are quite distinct from the ciliary apparatus of the 

 mammal. In teleosts a falciform process extends upward 

 from the floor of the retinal chamber. This has a small re- 

 tractor lentis muscle extending from its outer end which 

 pulls the lens backward to focus far objects. The lens is 



404 



THE NERVOUS SYSTEAA 



