THE SENSE OF VISION 



601 



Inserted into the middle portion of the tarsus of the upper lid are 

 fibres of the muscle which open the eye the levator palpebrae 

 superioris. 



The Cornea is the transparent layer in the front of the eyeball. 

 In the adult human being it is about 1 millimetre thick. It is the 

 first of the system of lenses of the eye-camera. It is continuous at its 

 edges with the sclerotic, forming the corneo-sclerotic or sclero-corneal 

 junction. The angle at which the cornea joins the iris within the 

 eyeball is known as the corneo-iridic or filtration angle. 



B.R 



Chor. 

 Ret. 



H.M 



O.N. 



FIG. 337. HORIZONTAL SECTION OF EYEBALL. (Parsons and Wright.) 



C., Cornea; Aq., aqueous humour; /., iris; S.L.L., suspensory ligament of lens; 

 C.P., canal of Petit; E.R., external rectus; Scl., sclerotic; Chor., choroid; Ret., 

 retina; H.M., hyaloid membrane; M.L., macula lutea; O.N., optic nerve; R.Av, 

 retinal artery; O.D., optic disc; I.R., internal rectus; Vit., vitreous humour; 

 C.B., ciliary body; C.Sch., canal of Schlemm; L., lens. 



Microscopically, the cornea consists of a layer of stratified 

 epithelium, in which ramify numerous delicate nerves, restiiig upon 

 a homogeneous layer (the anterior homogeneous layer of Bowman). 

 Beneath this layer the transparent fibres of the cornea proper are 

 arranged in layers, each successive layer being at right angles to the 

 next. Lying in minute tissue-fluid spaces between the fibres are 

 connective-tissue cells, known as the corneal corpuscles. In the 

 proper substance of the cornea there run delicate plexuses of nerve- 

 fibres derived from the ciliary nerves. Posterior to the cornea proper 

 comes a transparent elastic membrane (Descemet's), on the inner 

 surface of which is the single layer of flattened epithelial cells lining 



