276 OUTPOSTS OF THE INTELLIGENCE SERVICE 



Anatomy of Eye. 



To understand the mechanism of the eye even from the purely physical 

 standj^oint, i.e. as an optical instrument, it is necessary to have a clear 

 conception of its structure. 



Anatomy (contributed by J. Seeker). The human eyeball (Fig. 71) is a 

 hollow sphere of about 20 mm. in diameter. It consists of three concentrically 

 arranged coats enclosing a cavity containing three refracting media. These 

 coats are (i.) an outer fibrous envelope which is divided into an opaque portion, 

 the sclera, and a transparent portion, the cornea. The sclera constitutes the 

 posterior five-sixths of the coat, and the cornea, which has a greater convexity 

 than the sclera, the remaining anterior one-sixth, (ii.) A richly pigmented 

 coat, the chorioid, which is the vascular tunic of the eyeball, and contains 

 the intrinsic muscles of the eye, is the intermediate coat. At the junction 



5CLEKOTIC, 



OPTIC NERVEI 



Fig. 71. — Diagrammatic section tliroiigh c(juator of tlie left eye seen from above. 



of the sclera with the cornea, the chorioid ceases to be in intimate contact 

 with the sclera and projects as a curtain, the iris, into the cavity of the eye, 

 dividing the space between cornea and vitreous humour into an anterior and a 

 posterior chamber. In the centre of this curtain there is a central circular 

 aperture of variable size, the pupil. In the substance of the iris are two sets 

 of muscle fibres, one set, the constrictor muscles, arranged concentrically 

 with the pupil, and the other set, the dilator muscles, arranged radially. At 

 a point immediately posterior to the iris a series of about seventy radially 

 arranged processes project into the cavity. These projections or ciliary 

 processes consist of connective tissue containing blood vessels, and supplied 

 with muscular fibres from the ciliary muscle, which has its main body in the 

 chorioid coat at the region from which the ciliary processes originate. The 

 ciliary muscle arises from a spur of the sclera at the corneo-sclerotic junction 

 and consists of two sets of fibres, a circular and a radial set. The latter passes 

 into the ciliary processes, and together with them constitutes the ciliary 

 body, (iii.) The innermost coat, the retina, is the sensitive layer of the visual 



