SECTION IX 

 MOVEMENTS OF THE EYEBALLS 



IN order to obtain stinct vision of any object, an image of it must be 

 formed on the fovea centralis. Visual attention, i.e. the fixing of the gaze on 

 any object, involves the adjustment of the visual axes by movements of the 

 eyeball, in order that the image of the object of attention shall fall on the 

 central spot in each eye. 



The eyeball rests on a pad of fat surrounded by a denser capsule of connective 

 tissue, the capsule of Tenon, from which it is separated by a lymph space. Within 

 this space it is able to rotate around axes which pass through a point almost in the middle 

 of the eyeball. These movements of rotation are carried out by the six ocular muscles, 

 the four recti and the two oblique. The four recti muscles the superior, inferior, 

 external, and internal arise from a continuous tendinous oval rin^. which is attached 

 at the back of the orbit to the margin of the optic foramen and sphenoidal fksure. 

 From this common origin the muscles pass forwards as flat bands close to the walls 

 of the orbit ; they come in contact with the eyeball at its equator, passing through the 

 surrounding lymph space, and are attached to the eyeball about 7 mm. behind the 

 margin of the cornea, their positions being indicated by their names. Of these muscles 

 the internal rectus is the thickest and strongest, the superior rectus is the thinnest 

 and weakest. 



The superior oblique muscle arises in a short tendon attached to the back part of 

 the orbit in front of and internal to the optic foramen. The muscle runs forward in 

 the upper and inner angle of the orbit and ends in a round tendon, which passes through 

 a fibrous pulley at the upper and inner angle behind the anterior margin of the orbit. 

 The tendon then makes a sharp bend and passes outwards, backwards, and down- 

 wards between the superior rectus muscle and the eyeball, and is attached to the latter 

 just below the outer edge of the superior rectus, behind the attachments of the rectus 

 muscles and about half-way between the anterior and posterior poles of the eyeball. 



The inferior oblique muscle rises from the orbital plate of the superior maxilla, 

 just within the anterior margin of the orbit. The muscle forms a flat band and passes 

 upwards, backwards, and outwards between the inferior rectus and the wall of the orbit, 

 and ends in a tendinous expansion, which is inserted under the external rectus muscle 

 into the posterior and outer part of the eyeball, somewhat behind the line of attach- 

 ment of the superior oblique muscle. 



In discussing the actions of these muscles we may assume the eyes to be 

 in what is known as their primary position, i.e. with the visual axes parallel 

 and directed to a point on the horizon. It is evident 'that if the muscles 

 rotate the eyeball about an axis in a plane at right angles to the visual axes, 

 the pupils will move inwards, outwards, or in any direction, but will not 

 rotate. On the other hand, if the axis of rotation of a movement produced 

 by any muscle lies in a plane which is not vertical to the visual axes, then 



585 19* 



