THE EYEBALL. 



337 



mar plexus on the outer surface of the membrana propria. From this net- 

 work fibrils penetrate the basement membrane and end in close relation with 

 the gland-cells and muscle-elements. 



THE EYE. 



The organ of sight proper includes only the eyeball or globe of the eye; 

 with it, however, are closely associated other structures, as the eyelids, the 

 lachrymal apparatus, the orbital fascia and fat, and the ocular muscles, which 

 serve for its protection, support and change of axis. The structure of the 

 eyeball, therefore, will be described first; afterwards, that of some of the 

 accessory organs. 



THE EYEBALL. 



The human eyeball is an approximate sphere with an antero-posterior 

 diameter (24.2 mm.) of slightly less than one inch. It is, however, some- 

 what flattened from above downwards and from side to side. Its shape, 



Crystalline lens 



Suspensory ligament of lens \ 

 Canal of Schlemm 



Ciliary body 

 Conjunctiva 



Cornea 



Anterior chamber 

 Iris 

 Posterior chamber 



Sclero-corneal junction 



Internal muscle - 



Vena vorticosa -i-4 f 



Ciliary nerve 



Posterior ciliary vessels 



Hyaloid canal 



Optic nerve 



Central retinal'vessels 



External muscle 



Vitreous 



Sclera 



Fovea central is 

 v Cptic papilla 



Fie. 383. Diagrammatic horizontal section of right eye. X 2&- 



therefore, is spheroidal, with the vertical diameter (23.2 mm.) the shortest. 

 The eyeball consists of three concentric tunics or coats: (i) the external or 

 fibroiis tunic, composed of the sclera and the cornea; (2) the middle or 

 vascular tunic, which is pigmented, partly muscular and composed, from 

 behind forwards, of the choroid, the ciliary body, and the iris; and (3) the 

 inner or nervous tunic, usually called the retina, which is an expansion from 

 the brain and contains the nerve-cells, the nerve-fibres and the special neuro- 

 epithelium for the reception of the visual stimuli. Within these tunics are 

 enclosed the refracting media the aqueous humor, the crystalline lens, and 

 the vitreous body. 



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