210 



COMPARATIVE ANATOMY 





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the iris (Fig. 168), which retains in the centre a circular or slit-like 

 aperture, the fu^pil, through which the rays of light pass. The 

 amount of light admitted is regulated' by the dilator and con- 

 strictor (sphincter) muscles of the iris, which are able to increase 

 or lessen the size of the pupil ; the iris thus serves as a screen to 

 regulate the amount of light which enters the eye. 



Not only is the size of the pupil inconstant, but the lens is also 

 capable of undergoing considerable change in form, becoming more 

 flattened or more convex, as the case may be. The former con- 

 dition occurs when distant, the' 

 latter when near objects are 

 looked at. This delicate accom- 

 modating a-ffparatus is regulated 

 by a ciliary muscle, (tensor choro- 

 idem) supplied by the oculomotor 

 nerve, which arises in a circle all 

 round the eye from the point of 

 junction of the iris and sclerotic 

 and is inserted along the peri- 

 pheral border of the iris (Fig. 

 168). 



Externally to the vascular 

 layer of the choroid is a lymph- 

 sinus with pigmented walls 

 (lamina fusca) ; and externally to 

 this, again, is a firm fibrous, partly 

 cartilaginous, or even ossified 

 layer, the sclerotic. The latter 

 passes internally into the sheath 

 of the optic nerve, which is con- 

 tinuous with the dura mater, and 

 externally into the coriiea, the 

 outer surface of which is covered 

 over by an epithelial layer con- 

 tinuous with the epidermis — the 

 conjunctiva. The sclerotic and 

 cornea together form a firm outer 

 support for the eye, and thus, to- 

 gether with the gelatinous mass 

 of the vitreous humour, guarantee 

 the rigidity necessary for the physiological activity of the nerve 

 end-apparatus. Between the cornea and iris there is a large lymph- 

 space, the anterior chamher of the eye (Fig. 168), its contained 

 fluid being called the aqueous humour. Other lymph-spaces are 

 also present, e.g., between the choroid and sclerotic. 



The deep orbit, formed by the skull, serves as a further pro- 

 tection for the eye, as do also certain accessory structures, which 

 may be divided into three categories, viz. : — 



Fic4. 168. — Diagram OF A HoKizoNTAL 

 Section thbough the Left 

 Human Eye. (Seen from above.) 



Op, optic nerve ; OS, sheath of optic 

 nerve ; MF, blind-spot ; So, yellow 

 spot (fovea centralis) ; Bt, retina ; 

 PU, pigment epithelium of the 

 retina ; Ch, choroid, with its lamina 

 fnsca (Lj) and vascular layer (GS) ; 

 <Sc, sclerotic ; Co, cornea ; Cj, con- 

 junctiva ; MD, membrane of Uesce- 

 met ; C'lS, canal of Schlemm (the 

 dotted line should extend further 

 through the sclerotic to the small 

 oval aperture) ; Ir, iris ; Lc, ciliary 

 ligament ; 0, ciliary process ; VK, 

 HK, anterior and posterior chamber 

 of the ej'e ; L, lens ; H, hyaloid 

 membrane ; Z, Zone of Zinn, CP, 

 canal of Petit ; Cv, vitreous humour. 



