300 



ADAPTATIONS TO SPACE AND MOTION 



are placed so far laterally, and have so little mobility, that they are in 

 need of all possible means of converging their visual axes intra-ocularly. 

 The most conspicuous and common of all of these arrangements is 

 the static condition which may be termed 'nasad asymmetry', character- 

 istic of some marine (but not freshwater) fishes, many lizards, all birds, 

 ungulates and carnivores (e.g. cougar. Fig. 71, p. 173). It expresses itself 

 in a permanent anatomical tilting of the cornea and lens toward the 

 snout, so that a line through their centers (the true visual axis) strikes 

 the retina far temporally from its center. To carry out the asymmetry, 

 the ciliary body is usually shortened in the nasal quadrant, though some- 

 times the forward extension of the temporal portion of the retina restores 

 practical uniformity of width to the ciliary zone in all meridians. 



(C) Eye Movements and the Fovea 



Kinds of Eye Movements — Except where the eyeball is practically 

 microscopic (blind fishes, cave salamanders, etc.), the standard set of six 

 oculorotatory muscles is always present, even in animals whose eyes 

 might turn but never do, and even in those whose orbits are so snug that 

 the eyes cannot be turned even passively. Most eyes, of course, can turn 

 in their orbits; and their movements fall into a classification as follows: 



{Always coordinated, so as to appear 

 conjugated. (In ail vertebrates whose 

 eyes are mobile at all). 



Eye 

 movements 



Spontaneous 

 (voluntary) 



Independent - 



With no coordination. (In most 

 lizards and in birds) . 



With coordination in convergence. 

 (In some fishes and in chameleons). 



Conjugate (In mammals exclusively). 



Involuntary eye movements, in the sense implied here, are not neces- 

 sarily either unconscious or incapable of being inhibited, but they are 

 not willed movements made for the purpose of changing the visual field. 

 Rather, they are automatic, reflex movements which are intended to keep 

 the visual field as nearly constant as possible during locomotion and 

 during passive jogglings of the head and body. In this class fall the vari- 

 ous 'compensatory' and 'nystagmic' movements. An example par excel- 



