556 CYCLOSTOMES 



The lamprey orbit is not bounded, except in part, by the cranium, 

 but by a spherical connective-tissue capsule. The extra-ocular muscles 

 show some unique features, but none which could not — with a little 

 revision — be brought into line with the situation in other vertebrates. 

 They insert far forward, at the limbus, with some tendency to coalesce 

 there. The inferior oblique and the internal rectus originate together, 

 at a point farther nasally than the common point of origin (near the 

 optic nerve) of the other three recti. The inferior oblique and the in- 

 ternal and superior recti are supplied by the third cranial (oculomotor) 

 nerve, which in other vertebrates also innervates the inferior rectus (see 

 Fig. 70, p. 172), In lampreys however the sixth (abducens) nerve not 

 only supplies the external rectus as usual, but branches to the inferior 

 rectus as well. This nerve emerges from the brain unusually far forward, 

 and has been claimed to contain third-nerve fibers, which are perhaps 

 those which go to the inferior rectus. The superior oblique is identifiable 

 as such only by its nerve supply — from the fourth (trochlear) nerve — 

 for it has a unique location, and inserts on the ventro-temporal quadrant 

 of the eyeball. This has led some to refer to it as a 'posterior oblique', 

 and to suggest that it is not homologous with the superior oblique of 

 other vertebrate groups. 



The corneal muscle (of accommodation), which is also outside the 

 eyeball, is homologous with the oculomotor muscles inasmuch as it de- 

 velops from one or two of the cephalic myotomes. It inserts into the 

 skin of the spectacle which covers the cornea (Fig. 161). 



The eyeball, as in all groups of fishes, is flattened anteriorly so that 

 its antero-posterior axis is its shortest diameter. The major (equatorial) 

 diameter varies from about 1.5mm. in the smallest brook forms {e.g., 

 Ichthyomyzon fossor, Eudontomyzon cepypterus) to about 6.0mm. in 

 the larger parasitic petromyzonids {e.g., Entosphenus tridentatus, land- 

 locked Petromyzon marinus) and 7.0 mm. in Geotria australis. 



The virtual space between the dermal spectacle and the cornea is 

 occupied by a delicate mucoid tissue (thick in brook lampreys, thin in 

 larger forms, where it may be almost lacking under the center of the 

 spectacle), which belongs to neither structure, but is rather a continu- 

 uation of the lining of the orbital capsule. The sclera is a thin membrane 

 in, brook lampreys. In the larger parasitic species it is relatively and 

 absolutely thicker, and in the fundus may be as thick as the retina; but 

 it is always purely fibrous in structure, never with any embedded carti- 

 lage or bone. Such a sclera may of course descend from an ancestral 



