THE ORGANS OF SPECIAL SENSE 571 



ated, the nucleus lying at about the centre of the fibre near the axis 

 of the lens. The most central of the lens fibres are usually non- 

 nucleated. The fibres extend meridionally from before backward 

 through the entire thickness of the lens. They are united by a small 

 amount of cement substance. The lens is surrounded by the lens 

 capsule (Fig. 378, b), a clear homogeneous membrane which is about 

 i2« thick over the anterior surface of the lens, about half as thick 

 over the.posterior surface. Between the capsule and the anterior and 

 lateral surfaces of the lens is a single layer of cuboidal epithelial cells 

 (Fig. 378, c), the lens epithelium. Attached to the capsule of the lens 

 anteriorly and posteriorly are membrane-like structures which con- 

 stitute the suspensory ligament of the lens. These pass outward and 

 unite to form a dehcate membrane, the zonula ciliaris or zonule of 

 Zinn (Fig. 371). This bridges over the inequalities of the cihary 

 processes and, continuing as the hyaloid membrane, forms a lining for 

 the vitreous cavity of the eye. The triangular space between the 

 two layers of the suspensory ligament and the lens is known as the 

 canal of Petit. 



The vitreous body is a semifluid substance containing fibres which 

 run in all directions and a small number of connective-tissue cells and 

 leucocytes. Traversing the vitreous in an antero-posterior direction 

 is the so-called hyaloid or Cloquet's canal, the remains of the embry- 

 onic hyaloid artery (page 576). 



Blood-vessels. — The blood-vessels of the eyeball are divisible 

 into two groups, one group being branches of the central artery of the 

 retina, the other being branches of the ciliary artery. 



The central artery of the retina enters the eyeball through the 

 centre of the optic nerve. Within the eyeball it divides into two 

 branches, a superior and an inferior. These pass anteriorly in the 

 nerve-fibre layer, giving off branches, which in turn give rise to capil- 

 laries which supply the retina, passing outward as far as the neuro- 

 epithelial layer and anteriorly as far as the ora serrata. The smaller 

 branches of the retinal arteries do not anastomose. In the embryo 

 a third vessel exists, the hyaloid artery. This is a branch of the cen- 

 tral retinal artery and traverses the vitreous to the posterior surface 

 of the lens, supplying these structures. The hyaloid canal, or canal 

 of Cloquet, of the adult vitreous, represents the remains of the degen- 

 erate hyaloid artery (page 576). The veins of the retina accompany 

 the arteries. 



The cihary arteries are divisible into long ciliary arteries, short 



