THE MUSCLES OF THE EYEBALL. 421 



ball, on its opaque, or so-called sclerotic coat. The oblique muscles 

 are named, the one superior, the other inferior. The superior oblique, 

 4, 4, arises, like the recti muscles, from the border of the optic fora- 

 men, and passes forward to the upper and inner side of the orbit ; 

 there, it ends in a small tendon, which runs through a fibro-cartilagi- 

 nous pulley, or trochlea, attached to the bone in this situation, and 

 lined by a synovial membrane ; hence this muscle has received the 

 name of the trochlear muscle. From the pulley, the tendon of the 

 superior oblique muscle is reflected backwards and outwards, to be 

 inserted into the sclerotic coat, on the upper surface of the eyeball, a 

 little behind its middle. The inferior oblique muscle arises from a 

 depression in the inner and fore part of the floor of the orbit, passes 

 outwards and backwards, beneath the eyeball, and is inserted into the 

 sclerotic coat, upon the outer and posterior surface of the globe. 



The straight muscles are so attached, that they can turn the eye- 

 ball upwards, downwards, inwards, or outwards, according to the mus- 

 cle brought into play ; hence they have been named respectively, the 

 attollens, depressor, adductor, and abductor muscles of the eyeball. If 

 two adjoining recti muscles act together, the eyeball is carried in an 

 intermediate or oblique direction. When all four muscles act simul- 

 taneously, the eyeball must be strained backwards, and some have 

 supposed that by this action, the antero-posterior diameter of the eye- 

 ball may be increased. When in a state of rest, the elasticity of the 

 surrounding structures, keeps the eyeballs in their parallel position, 

 and this parallelism is accurately maintained in many of its move- 

 ments. But if one muscle becomes weaker than its antagonist muscle, 

 or obtains an undue preponderance over it, the natural position of 

 equilibrium is destroyed, and the distortion named strabismus or squint, 

 either internal, or external, for example, is produced. The oblique 

 muscles rotate the eye on its antero-posterior axis, the superior oblique 

 rolling its upper half inwards, the inferior oblique rolling its lower 

 half inwards. These two muscles, being inserted behind the transverse 

 axis of the eyeball, also turn its anterior surface outwards and down- 

 wards, when the superior oblique acts alone, and outwards and up- 

 wards, when the inferior oblique acts alone. Their combined action 

 turns the anterior surface of the ball directly outwards. 



The upward and downward movements of the eyeballs, are more 

 rapid than those from side to side, or than the oblique movements ; 

 the motions, which are very perfectly under the control of the will, 

 are so rapid as to be singly immeasurable, but by repeating them over 

 several times consecutively, in each direction, the difference is multi- 

 plied, and becomes easily noticeable. All these movements are more 

 easy of execution, if they are performed from the natural, or, as it 

 were, instinctive position of rest of the eyeballs that is, with the optic 

 axes directed horizontally forwards, and in parallel lines. The pri- 

 mary or simple motions of the eyeballs, may be referred to rotations 

 around three principal axes viz., the antero-posterior axis, the trans- 

 verse axis, and the vertical axis. The movements around the vertical 

 axis are performed by the external and internal recti ; those around 

 the transverse axis, by the superior and inferior recti, aided respec- 



