THE VITREOUS BODY. 1473 



passage of nutritive fluid. The fibres are so arranged that their ends terminate along 

 definite radiating striae, or lens-stars, which in the young lens are three in number 

 on each surface. In the adult lens additional rays increase the number to from six to 

 nine, the striie being less distinct but distinguishable with the ophthalmoscope. The 

 lens-fibres which come from the pole of one surface of the lens terminate at the end of 

 one of the radial stride in the other, and conversely ; the intervening fibres take up 

 intermediate positions. In adult life the lens-fibres become more condensed, the lens 

 loses its clear appearance, and assumes a yellowish tint. This change affects the 

 nucleus first and the periphery later, coincidently the lens becoming less elastic as 

 the result of its loss of water. 



Practical Considerations. — The lens may be congenitally absent (aphakia), 

 or it may be abnormal in size, shape, position, or transparency. Its anterior or 

 posterior surface may be abnormally convex (lenticonus). Congenital anomalies of 

 position (ectopia lentis) occur rarely. The lens may remain in its foetal position in 

 the vitreous chamber, or it may be displaced in an equatorial direction from faulty 

 development and weakness of some part of the suspensory ligament. This weakness 

 usually occurs below so that the lens mox^es upward. The ligament may be absent in 

 its whole circumference, when the lens may be protruded into the anterior chamber. 



Coloboma or partial deficiency of the lens is very rare. It is with comparative 

 frequency associated with a similar defect in the iris, ciliary body and choroid, and, 

 like it, is usually in the lower portion. A defect of the corresponding part of the 

 suspensory ligament is occasionally present. 



Traumatic luxation of the lens may take place into the vitreous or aqueous 

 chamber. It may occur laterally through the coats of the eyeball into the capsule 

 of Tenon or under the conjunctiva. That into the vitreous is most frequent. 



The capsule of the lens is strong and elastic. It is at the same time brittle, 

 breaking like thin glass when torn as by a sharp instrument. For this reason it is 

 sometimes called the vitreous membrane. The anterior layer of the capsule is con- 

 siderably thicker than the posterior, and is more liable to pathological changes, pro- 

 ducing opacities. Wounds of the capsule permit the aqueous fluid to reach the lens 

 fibres, which then become swollen, opaque, and finally disappear from the dissolving 

 action of the aqueous. Advantage of this is taken in the needling operation (dis- 

 cission) for the removal of a cataract. 



In children the lens substance is of nearly equal consistency throughout, but as 

 age advances the central portion becomes gradually more condensed, and is called 

 the nucleus. A well-marked nucleus, however, does not exist until adult life. In 

 old age the lens loses its elasticity so that the changes necessary for accommodation 

 are interfered with, and sight is disturbed. The hardened nucleus permits a greater 

 reflection of light than the outer portion, so that the lens is more readily seen in 

 older people, and the pupil loses more or less its blackness. 



A cataract is an opacity of the lens, or its capsule, but that of the lens is so 

 much more common than that of the capsule, that by the word cataract the*lenticular is 

 usually meant, unless the word is otherwise qualified. All cataracts are at sometime 

 partial, and they are called according to their location, anterior polar or capsular, 

 posterior polar or capsular, central or nuclear, lamellar, perinuclear and cortical. 

 Cataract occurs sometimes in the young, and is then soft ; that is, the lens has no 

 nucleus. 



The Vitreous Body. 



The vitreous body (corpus vitreum) fills the space between the lens and the 

 retina, being in close contact with the retina and acting as a support to it as far 

 forward as the ora serrata. Here it becomes separated from the retina and passes to 

 the posterior surface of the lens, presenting a shallow depression, the fossa hya- 

 loidea or patellar fossa, on its anterior surface for the reception of the lens. The 

 fresh vitreous is a semifluid, perfectly transparent mass which consists of about 98.5 

 per cent, of water. 



The structure of the vitreous has been a subject of protracted dispute, but 

 recent investigations have established beyond question that it possesses a framework, 



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