SECTION II 



EYE MOVEMENTS 

 ANATOMY OF THE ORBIT 



THE eyeball and its accessory structures lie in the bony orbital cavity, 

 surrounded and padded by a mass of semiliquid fat. The cavity is pierced 

 by several apertures, through which pass various vessels and nerves. The 

 optic nerve enters through an aperture of its own, the optic foramen, together 

 with the ophthalmic artery. Most of the other nerves and vessels concerned 

 with vision pass through the sphenoidal fissure. These are the 3rd, 

 4th and 6th motor nerves which innervate the muscles controlling eye 

 movement, sensory branches of the upper division of the 5th nerve, 

 connected with the cornea, conjunctiva, lids, etc., and the ophthalmic 

 veins. In order to allow the eye free movement the surrounding structures 

 form with it a ball-and-socket joint. The joint cavity is formed by a pouch- 

 shaped structure called the capsule of Tenon. This pouch surrounds the 

 posterior four-fifths of the eyeball, in fact its folded margin touches the 

 ocular conjunctiva. The pouch is made of a tough smooth membrane, and 

 contains synovial fluid so as to allow the eye the greatest freedom 

 of movement. Since the six muscles which cause the eye movements are 

 attached to the bony wall of the orbit behind and to the front portion of 

 the globe in front, it is clear that the tendons of the muscles must pierce 

 the capsule. This is done in a very admirable manner, so as to allow free 

 movement and at the same time to prevent escape of synovial fluid. Moreover 

 the edges of the apertures, through which the tendons enter, form strong, 

 bands which are attached to the bony walls of the orbit. These bands act 

 as check ligaments, preventing excessive movement on the part of the 

 muscles. Tenon's capsule contains numerous smooth muscle fibres which 

 are innervated by sympathetic nerves from the cavernous plexus, via the 

 ciliary (lenticular) ganglion and the long ciliary nerves. Stimulation of the 

 nerves described causes contraction of these muscle fibres, protrusion of the 

 eyes and rise of intraocular pressure. But the most important function of 

 these fibres is that by their tone they prevent the eye from being dragged 

 back into the socket by the contraction of the external muscles. One of the 

 explanations of the protrusion of the eyes in exophthalmic goitre is given 

 to be the stimulation of the sympathetic nerves in the neck by the local 

 pressure of the thyroid tumour, and it is said that removal of the superior 

 cervical ganglion relieves the condition. 



With regard to the position of the centre of rotation of the eye it might 



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