CHAPTER XXVIII 



MOVEMENTS OF THE EYEBALL PARTS FOR PROTECTION OF 



THE EYE 



Action of the recti muscles Action of the oblique muscles Associated action of the muscles 

 of the eyeball Centres for vision Perception of colors Parts for the protection of the 

 eyeball Muscles that open and close the eyelids Conjunctival mucous membrane 

 The lachrymal apparatus The tears. , 



THE eyeball nearly fills the cavity of the orbit, resting by its poste- 

 rior portion upon a bed of adipose tissue, which is never absent, even in 

 extreme emaciation. External to the sclerotic, is a fibrous membrane, 

 the tunica vaginalis oculi, or capsule of Tenon, which is useful in main- 

 taining the equilibrium of the globe. This membrane surrounds the 

 posterior two-thirds of the globe and is loosely attached to the sclerotic. 

 It is perforated by the optic nerve posteriorly, and by the tendons of 

 the recti and oblique muscles of the eyeball in front, being reflected 

 over these muscles. It also is continuous with the palpebral ligaments 

 and is attached, by two tendinous bands, to the border of the orbit at 

 the internal and the external angles of the lids. 



The muscles that move the globe are six in number for either eye. 

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

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

 arise posteriorly from the apex of the orbit. The recti pass directly 

 forward by the sides of the globe and are inserted by short tendinous 

 bands into the sclerotic, at a distance of one-fourth to one-third of an 

 inch (6.4 to 8.5 millimeters) from the margin of the cornea. The supe- 

 rior oblique, or trochlearis muscle passes along the upper and inner wall 

 of the orbit to a point near the inner angle. It here presents a rounded 

 tendon, which passes through a ring, or pulley, of fibro-cartilage ; and 

 it is from this point that its action is exerted on the globe. From the 

 pulley, or trochlea, the tendon becomes flattened, passes outward and 

 backward beneath the superior rectus, and is inserted into the sclerotic 

 about midway between the superior and the external rectus and just 

 behind the equator of the globe. The inferior oblique muscle arises 

 just within the anterior margin of the orbit near the inner angle of the 

 eye and passes around the anterior portion of the globe beneath the 

 inferior rectus and between the external rectus and the eyeball, taking a 

 direction outward and slightly backward. Its tendon is inserted into 



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