262 ADAPTATIONS TO SPACE AND MOTION 



dermal retractor lentis muscle, sometimes called the 'campanula (or 

 plumula) Halleri', The other tendon of this muscle originates tem- 

 porally and cranially in the anterior end of the falciform (= sickle- 

 shaped) process where such is present. The falciform process is a 

 ridge in the floor of the eyeball, running from behind and temporally 

 (near, at, or even from above the disc) forward and nasally along more 

 or less of the length of the original course of the embryonic fissure of 

 the optic cup (see Chapter 5, section A) , The falciform process may be 

 most simply (though not too accurately) described as a herniation of 

 the chorioid up through the unclosed fissure. It is lacking in many fishes 

 without much regard to their taxonomic positions, and its place in the 

 internal nutritional system of the eye is always taken by a system of 

 ('hyaloid') blood vessels spread out in a thin membrane at the vitreo- 

 retinal interface. In fishes which lack the falciform process, the lens- 

 muscle is kept but its fixed anchorage punctures the retina near the ora. 



The nasoventral attachment of the lens-muscle, and its orientation, 

 result in a backward (craniad) and temporad duction of the lens upon 

 contraction. The impulses to contraction come over a large branch 

 of the oculomotor nerve which runs along through the chorioid be- 

 neath the falciform process and, accompanied by a blood vessel, de- 

 parts from the process anteriorly and runs free through the ocular cavity 

 to reach the little muscle. The movement of the lens is roughly opposite 

 to that accomplished in the elasmobranch eye by the protractor lentis, 

 and accommodates the teleost eye for distance instead of for near. 



Except in the tubular eyes of many deep-sea teleosts (whose lenses 

 can only be moved slightly backward, if at all) , the lens moves laterally, 

 toward the fish's tail, rather more than it moves backward into the retinal 

 cup. This is particularly true in species which have a fovea (see p. 304). 

 During accommodation, a teleost's attention is obviously upon the image 

 in the temporad periphery, which is the location of the area centralis and 

 is the part of the retina involved in binocular vision. 



The retractor lentis muscle in its various manifestations undoubtedly 

 does all of the actual work of accommodation in the teleost eye. How- 

 ever, in this group a ciliary muscle is first seen. It is so very small that 

 it makes no bulge in the ciliary region of the uveal tract; and, there being 

 no ciliary folds (except a few dorsal and ventral ones in a few species, as 

 also in the rays among the elasmobranchs) there is really no discrete 

 ciliary body at all. The uvea is much alike from ora terminalis to pupil 

 (Fig. 67, p. 159), and unless one calls this whole region 'iris' one must 



