390 PHYSIOLOGY CHAP. 



the sides ; the lateral border recedes backward, so that the 

 temporal side of the front half of the bulb is practically un- 

 covered. In eyes of normal size the line that joins the two outer 

 edges of the orbit passes approximately through the rotation- 

 centres of both eyes (line x'x of figure). 



The hollow space which contains the eyeball is formed behind 

 by Tenon's capsule and the peribulbar and retrobulbar cushions 

 of fat, in front by the inner surface of the lids and the conjunctival 

 sac. Attached only to the optic nerve, like a fruit to its stalk, 

 the eye can be rotated in its cavity by the recti muscles in four 

 directions, corresponding to the four sides of the orbit ; but owing 

 to the incompressibility of the solid and liquid tissues which 

 surround it, it is but little displaced. 



Tenon's capsule, according to Motais (1887), is an aponeurotic 

 membrane which takes origin in the periosteum of the optic 

 foramen and is attached in front to the margin of the orbit ; it 

 provides a sheath for the external ocular muscles, the optic nerve 

 and the sclerotic (Fig. 182, right). The inner surface of the 

 capsule is attached to the sclerotic only by delicate bundles of 

 connective tissue, and is, for the most part, separated from it by 

 a wide lymphatic space, so that when the eye moves it serves as a 

 synovial capsule. 



The sheaths which the capsule provides for the recti muscles 

 are thicker, and adhere more closely to the substance of the 

 muscles near their tendinous attachments to the eyeball ; 

 ligamentous expansions from their outer sides are attached to 

 the margin of the orbit (aa r of figure) ; at the inner sides they are 

 continuous with a semi -lunar thickening of the capsule, the 

 ligamentum capsulare (c of figure). These elastic and distensible 

 bands are of importance, since by fixing the eyeball anteriorly 

 they limit the effect of traction of the muscles and act as 

 regulators of the movements of the eye when its muscles contract 

 (Motais). After cutting one of the moderator ligaments the eye- 

 ball can make wider excursions to the corresponding side with 

 less effort (Merkel). 



The muscles that effect the movements of the eye are attached 

 in front to the equator of the eyeball, like the bridle to a horse's 

 head, and behind to the bony walls of the optic foramen. There 

 are four recti and two obliqui muscles. The anterior tendinous 

 attachments of the recti muscles are often arranged spirally, 

 according to Tillaux, since the rectus internus is inserted about 

 5 mm. from the edge of the cornea, the rectus inferior 6, the 

 rectus externus 7, and the rectus superior 8 mm. The internal 

 rectus is the broadest and strongest, the superior the smallest and 

 weakest. The superior oblique or trochlear muscle takes origin 

 from the inner part of the optic foramen, runs along the inner 

 wall of the orbit, and passes as a small round tendon through a 



