THE SENSE OF VISION. 299 



at right angles to it, as in extremely myopic or short-sighted eyes, the rotation 

 of the eyeball may be considerably limited in its extent. In addition to the 

 movements of rotation round a centre situated in the axis of vision, the eye- 

 ball may be moved forward and backward in the socket to the extent of about 

 one millimeter. This movement may be observed whenever the eyelids are 

 widely opened, and is supposed to be effected by the simultaneous contraction of 

 both the oblique muscles. A slight lateral movement has also been described. 



The movements of the eye will be best understood when considered as 

 referred to three axes at right angles to each other and passing through the 

 centre of rotation of the eye. The first of these axes, which may be called 

 the longitudinal axis, is best described as coinciding with the axis of vision 

 when, with head erect, we look straight forward to the distant horizon ; the 

 second, or transverse, axis is defined as a line passing through the centres of 

 rotation of the two eyes; and the third, or vertical, axis is a vertical line nec- 

 essarily perpendicular to the other two and also passing through the centre of 

 rotation. When the axis of vision coincides with the longitudinal axis, the eye 

 is said to be in the primary position. When it moves from the primary posi- 

 tion by revolving around either the transverse or the vertical axis, it is said to 

 assume secondary positions. All other positions are called tertiary positions, 

 and are reached from the primary position by rotation round an axis which 

 lies in the same plane as the vertical and horizontal axis — i. e. in the " equato- 

 rial plane" of the eye. When the eye passes from a secondary to a tertia re- 

 position, or from one tertiary position to another, the position assumed by the 

 eye is identical with that which it would have had if it had reached it from 

 the primary position by rotation round an axis in the equatorial plane. In 

 other words, every direction of the axis of vision is associated with a fixed 

 position of the whole eye — a condition of the greatest importance for the easy 

 and correct use of the eyes. A rotation of the eye round its antero-posterior 

 axis takes place in connection with certain movements, but authorities differ 

 with regard to the direction and amount of this rotation. 



Muscles of the Eye. — The muscles of the eye are six in number — viz: 

 the superior, inferior, internal and external recti, and the superior and inferior 

 oblique. This apparent superfluity of muscles (for four muscles would suffice 

 to turn the eye in any desired direction) is probably of advantage in reducing 

 the amount of muscular exertion required to put the eye into any given posi- 

 tion, and thus facilitating the recognition of slight differences of direction, for, 

 according to Fechner's psycho-physic law the smallest perceptible difference in 

 a sensation is proportionate to the total amount of the sensation. Hence if the 

 eye can be brought into a given position by a slight muscular effort, a change 

 from that position will be more easily perceived than if a powerful effort were 

 necessary. 



Each of the eye-muscles, acting singly, tends to rotate the eye round an axis 

 which may be called the axis of rotation of that muscle. Now, none of the 

 muscles have axes of rotation King exactly in the equator of the eve — i. e. 

 in a plane passing through the centre of rotation perpendicular to the axis 



