THE EYE 215 



the ciliary body. At the point of entrance of the optic nerve, 

 there are no nerve-cells in the retina, which is only represented 

 by an incomplete layer, termed the lamina cribrosa (see 

 Ophthalmoscopic Examination). 



The Vitreous Body, which occupies the posterior four- 

 fifths of the eyeball, consists of the vitreous humor enclosed 

 within a capsule, termed the hyaloid membrane. The Crystal- 

 line Lens lies in a depression on the anterior aspect of the 

 vitreous body, and it is connected to the hyaloid membrane 

 by a suspensory ligament, which extends from the membrane 

 beyond the periphery of the lens to the anterior aspect of the 

 lens, where it blends with the capsule (Fig. 82). Circular in 

 shape, the lens possesses a diameter of about 10 mm., while 

 it is 4 mm. in thickness. It consists of concentric laminae of 

 highly specialised fibrous tissue enclosed within an elastic 

 capsule. When the eye is at rest, the anterior surface of the 

 lens is not so convex as the posterior surface, but, when the 

 meridional fibres of the ciliary muscle contract, the suspensory 

 ligament, which is adherent to the ciliary processes, is drawn 

 forwards and, owing to the elasticity of its substance, the lens 

 becomes more convex on its anterior surface. As age advances, 

 the tissue of the lens becomes denser and loses its elasticity, 

 thus accounting for the condition of presbyopia. When the 

 pupil of a presbyopic subject is examined obliquely in reflected 

 light, the appearance is suggestive of cataract, but ophthalmo- 

 scopic examination will be sufficient to show that the media 

 are quite translucent. 



Congenital malposition of the crystalline lens is a rare 

 abnormality and it is due to failure in development of the 

 suspensory ligament. 



The normal lens is perfectly translucent and, therefore, can 

 contain no blood-vessels. During the period of its formation, 

 however, the lens is supplied with blood by a small branch of 

 the ophthalmic artery, which passes forwards from the porus 

 opticus (optic disc) in a small canal in the vitreous body. 

 This artery disappears during the fifth month of mtra-uterine 



