258 



ADAPTATIONS TO SPACE AND MOTION 



These three kinds of intra-ocular movements are nearly always accom- 

 plished entirely by intra-ocular muscles. Extra-ocular ones have been 

 suspected in some cases of helping to alter the lens-retina distance by 

 deforming the eyeball in a regular manner. The mechanisms involved 

 are described below, and are summarized in Table VIII (pp. 272-3), 

 which should be consulted during the reading of the remainder of this 

 Section. 



?5 



Fig. 103 — The eye and surrounding structures in a lamprey, Lampetra fluviatilts, in hori- 

 zontal section; the anterior end of the animal is to the left. Modified from Franz. 



av- anterior surface of vitreous; c- cornea; er- external rectus; io- inferior oblique; ir- internal 

 rectus; n- optic nerve; s- spectacle (a 'window' in the head skin); sk- skin; sp- space between 

 speaacle and cornea; sr- superior reaus; /- tendon of cornealis muscle, inserting into spectacle; 

 V, V- venous sinuses which cushion eyeball. 



Lampreys — Inserted into the rim of the primary spectacle (see p. 449/) 

 at one side is the tendon of a massive muscle (Fig. 103) which lies in the 

 head outside of the orbital capsule and caudad from the eye, and repre- 

 sents portions of two myotomes. When this 'musculus cornealis' is con- 

 tracted, the spectacle is drawn taut and flattens the cornea. Since the 

 lens touches the latter on the inside, it is pressed backward and nearer 

 to the retina. The near-point of the resting eye is very close — about five 

 inches, for the eye is eight diopters myopic. In accommodation the eye 

 of course becomes emmetropic and may go on into a fairly high degree 

 of hypermetropia. 



Accommodation in the cyclostomes is thus accomplished by deform- 

 ing the eyeball from outside. The return to the resting shape is effected 



