vi DIOPTKIC MECHANISM OF THE EYE 275 



in a regular stellate, radial direction (Fig. 112). A membrane is 

 attached to both the anterior and the posterior surface of the 

 capsule of the lens ; these two membranes converge towards the 

 ciliary processes and unite with them, forming the two layers of 

 the so-called ligamentum suspensorium lentis, or zonule of Zinn. 

 It is generally held that this ligament arises from the doubling 

 of the hyaloid membrane. The anterior, more highly developed 

 layer follows the inflexions and eminences of the ciliary processes, 

 and forms the prolongation of the zonule of Zinn and the inner 

 limit of the posterior chamber. The posterior layer is continued 

 from the more flattened surface of the ciliary processes over the 

 anterior surface of the vitreous body. The circular space, 

 triangular in cross-section, enclosed between the two layers, is 

 known as Petit's canal. The hyaloid membrane, being united to 

 the uvea, follows the movements imparted to the latter by the 

 ciliary muscle. 



The space taken up by the aqueous humour is, as we have 

 seen, divided by the iris into an anterior and a posterior chamber. 

 The latter communicates by small slits in the anterior layer of 

 the suspensory ligament of the lens with the canal of Petit. The 

 aqueous humour is a clear fluid of lymphoid nature, which 

 contains a few leucocytes. It is probably secreted by the 

 epithelium of the ciliary body and its glandular crypts. It 

 communicates by means of the slits in the ligamentum pectinatum 

 iridis with the lymph spaces and the canal of Schlemm. 



In order to understand the blood-supply of the eye we may 

 profitably study Leber's diagram (Fig. 113). In it can be 

 distinguished the short and long posterior ciliary arteries, which 

 perforate the sclerotic near the entrance of the optic nerve ; the 

 anterior ciliary arteries, which perforate the sclera near the points 

 of attachment of the outer muscles of the eye ; and the large 

 vorticose veins, which leave the sclera behind the equator of the 

 eye. These vessels supply the different parts of the uvea as well 

 as the sclera. The cornea is entirely destitute of blood-vessels. 

 The course of the arteries and veins in the choroid, as shown by 

 Fig. 114, is characteristic. The vascular supply of the retina is 

 distinct from that of the uvea; it consists of the ramifications 

 of the central artery and veins, which run along the axis of the 

 optic nerve, as seen in Fig. 111. The two vascular systems com- 

 municate by means of anastomoses at the entrance of the optic 

 nerve. 



The outer and middle coats of the eye are supplied by 

 countless nerves, which give them sensibility and innervate the 

 ciliary muscles and muscles of the iris, as well as the muscle- 

 fibres of the blood-vessels. These are the ciliary nerves, derived 

 from the ciliary ganglion, which lies in the posterior part of the 

 orbit (Fig. 115). The ciliary ganglion receives a sensory root 



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