590 MANUAL OF PHYSIOLOGY. 



One of the most important means employed to enable us to 

 form accurate visual perceptions is the varied motion which the 

 eyeballs are capable of performing. 



MOVEMENTS OF THE EYEBALLS. 



The eyeballs may be regarded as spherical bodies, lying in loosely 

 fitted sockets of connective tissue padded with fat, in which they 

 can move or revolve freely in all directions, in a limited degree. 

 The muscles which act directly on the eyeball are six in number. 

 Four recti muscles, which pass from the back of the orbit and 

 are attached to the eyeball, one at each side and one above and 

 below, not far from the cornea, can move the front of the eye to 

 the right or left, and up or down respectively ; and two oblique 

 muscles which pass nearly horizontally outwards, and a little 

 backwards, and are attached to the upper and under surface of 

 the eyeball respectively. These muscles can slightly rotate the 

 eye on its antero-posterior axis, the upper one drawing the upper 

 part of the eyeball inwards, and the lower one, as its antagonist, 

 drawing the lower part inwards, so as to rotate the eyeball in an 

 opposite direction on the same axis. 



The internal and external recti draw the centre of the cornea 

 towards or from the median line respectively, directly opposing 

 one another. 



On account of the direction of the superior and inferior recti 

 being different from that of the axis of the eyeball, they draw 

 the outer edge of the cornea, not its centre, up and down respec- 

 tively, and at the same time tend to give the eyeball a slight ro- 

 tation in the same direction as the corresponding oblique mus- 

 cles. The tendency to rotation is counteracted by the antago- 

 nistic oblique muscle when simple elevation or depression is per- 

 formed. 



Thus pure abduction or adduction only requires the unaided 

 action of the internal or external recti, while direct depression 

 of the eye requires the combined action of the inferior rectus 

 and superior oblique, and direct elevation requires the superior 

 rectus and inferior oblique to act together. The various oblique 



