FIBKOUS TUNIC OF THE EYE. 807 



its equator, while its posterior two-thirds are enveloped by a loose membrane 

 termed the fascia bulbi (O.T. capsule of Tenon). 



The bulb of the eye- is not quite spherical, being composed of the segments of 

 two spheres, viz., an anterior, transparent, corneal segment, possessing a radius of 7 or 

 8 mm., and a posterior, opaque, scleral segment, with a radius of about 12 mm. 

 (Fig. 677). The anterior or corneal segment, in consequence of its shorter radius, 

 projects forwards, in front of the scleral portion, the union of the two parts 

 being indicated, externally, by a slight groove, the sulcus sclerse. The central 

 points of the anterior and posterior curved surfaces of the bulb constitute, 

 respectively, its anterior and posterior poles, and a straight line joining the two 

 poles is termed the optic axis ; an imaginary line encircling the bulb, midway 

 between the poles, is named the equator. The axes of the two bulbs are almost 

 parallel, diverging only slightly in front ; but the axes of the optic nerves converge 

 behind, and, if prolonged backwards, would meet in the region of the dorsum 

 sellse of the sphenoid. The sagittal and transverse diameters of the bulb are 

 nearly equal about 24 mm.; its vertical diameter is about 23'5 mm. All three 

 diameters are rather less in the female than in the male, but the size of the bulb 

 is fairly constant in the same sex. What are popularly described as large eyes 

 owe their apparent size to a greater prominence of the bulb and to a wider fissure 

 between the eyelids. 



At birth the bulb of the eye is nearly spherical and has a diameter of about 

 17'5 mm. By the age of puberty this has increased to 20 or 21 mm., after which it 

 rapidly reaches its adult size. 



Fascia Bulbi. The fascia bulbi (O.T. capsule of Tenon) is a fibrous tunic 

 enveloping the posterior two- thirds of the bulb of the eye, and separating the 

 posterior part of the bulb from the surrounding orbital fat. It blends posteriorly 

 with the sheath of the optic nerve and with the sclera around the lamina cribrosa ; 

 anteriorly it is continued into the ocular conjunctiva, and is also attached to the 

 ciliary region of the bulb. It is pierced by the tendons, of the ocular muscles, and 

 is reflected on each as a tubular sheath. The sheath on the tendon of the obliquus 

 superior surrounds the tendon as far as its pulley, to which it is attached ; that 

 on the obliquus inferior is prolonged as far as the floor of the orbit. The sheaths 

 on the recti muscles are continuous posteriorly with the perimysium of those 

 muscles, and each gives off an expansion. The expansion from the sheath of the 

 rectus superior blends with the sheath of the levator palpebrse superioris, and that 

 from the sheath of the rectus inferior is attached to the tarsus of the inferior 

 eyelid. The expansions from the sheaths of the medial and lateral recti are strong, 

 especially that from the latter muscle, and are attached to the lacrimal and zygo- 

 matic bones respectively ; they are named the medial and lateral check ligaments, 

 because they probably limit the action of the corresponding muscles. The portion 

 of the fascia bulbi which lieg inferior to the bulb of the eye has been named the 

 suspensory ligament (Lockwood) ; it is expanded in the centre, and is slung like 

 a hammock from side to side, its narrow ends being fixed to the lacrimal and 

 zygomatic bones. 



The bulb of the eye (Fig. 677) consists of three concentric tunics or coats, and 

 contains three transparent refracting media. The three tunics are : (1) an outer 

 fibrous tunic, consisting of an opaque posterior part, the sclera, and a transparent 

 anterior portion, the cornea ; (2) an intermediate vascular, pigmented, and partly 

 muscular tunic, the tunica vasculosa oculi, comprising, from behind forwards, the 

 chorioid, the ciliary body, and the iris ; (3) an internal nervous tunic, the retina. 

 The three refracting media are named, from before backwards, the aqueous humour, 

 the crystalline lens, and the vitreous body. 



TUNICA FIBEOSA OCULI. 



Sclera. The sclera is a firm, opaque membrane, forming approximately the 

 posterior five-sixths of the outer tunic. Thickest posteriorly (about 1 mm.), it thins 

 at the equator to 04 or 0'5 mm., and again increases to 0'6 mm. near the sulcus 



526 



