VASCULAE TUNIC OF THE EYE. 813 



pars ciliaris retinae ; this epithelium is invaginated to form more or less tubular 

 glands. 



M. Ciliaris. The ciliary muscle is triangular on horizontal or vertical section, 

 and consists of two sets of fibres meridional and circular (Fig. 683). The meridional 

 fibres arise from the scleral spur, already described, and radiate backwards, to be 

 attached to the ciliary processes and orbiculus ciliaris. When they contract the 

 chorioid is drawn forwards and the lens becomes more convex, owing to the 

 relaxation of its suspensory ligament (see p. 810). The circular fibres form a 

 triangular zone behind the filtration angle, close to the periphery of the iris. 

 Considerable individual differences are found as to the degree of development 

 of these two portions of the ciliary muscle ; the meridional fibres are always more 

 numerous than the circular fibres, the latter being absent or rudimentary in myopic 

 eyes, but well developed, as a rule, in hypermetropic eyes. 



Iris. The iris forms a contractile diaphragm in front of the lens, and is 

 pierced, a little to the nasal side of its centre, by an almost circular aperture, 

 the pupil, which, during life, is continually varying in size in order to regulate 

 the amount of light admitted into the interior of the eye. It partially divides 

 the space between the cornea and lens into two portions, which are filled by the 

 aqueous humour, and are named, respectively, the anterior and posterior chambers 

 of the eye. It is thinnest at its peripheral or ciliary margin which is directly 

 continuous with the ciliary body, and, through the medium of the ligamentum 

 pectinatum iridis, with the posterior elastic lamina of the cornea. Its pupillary 

 or free margin forms the circumference of the pupil, and rests upon the anterior 

 surface of the capsule of the lens. 



The distinctive colour of the eye, in different individuals, depends on the arrangement 

 of the pigment in the iris; in the blue eye the pigment is limited to the posterior 

 surface of the iris, but in the brown or black eye it is also scattered throughout its stroma ; 

 in the albino the pigment is absent. 



The pupil is closed, during the greater part of foetal life, by a thin transparent 

 vascular membrane, the membrana pupillaris, continuous with the pupillary margin of 

 the iris. Its vessels are derived partly from the vessels of the iris and partly from those 

 of the capsule of the lens ; they converge towards the middle of the membrane, near which 

 they form loops so as to leave the central part non-vascular. About the seventh month 

 the vessels begin to be obliterated, from the centre towards the circumference ; and this 

 is followed by a thinning and absorption of the membrane, which becomes perforated by the 

 aperture of the pupil. This perforation gradually enlarges, and at birth the membrane 

 has entirely disappeared ; in exceptional cases it persists. 



On the anterior surface of the iris is a layer of flattened endothelium, placed 

 on a basement membrane, and continuous with the endothelium of the anterior 

 chamber. Depressions or crypts are seen here and there in which the endothelium 

 and basement membrane are absent, and are, by some, regarded as stomata, 

 through which the lymph vessels of the iris communicate with the cavity of the 

 anterior chamber. The posterior surface of the iris is covered with a basement 

 membrane, on which are placed two layers of columnar, pigmented epithelium, 

 continuous with the pars ciliaris retinae, and termed the pars iridica retinas. The 

 stroma iridis, or proper tissue of the iris, consists of delicate connective tissue 

 and elastic fibres, with pigmented cells, blood-vessels, nerves, and non-striped 

 muscle. 



The blood-vessels of the iris (Fig. 682) are derived from the long ciliary and 

 the anterior ciliary arteries. The long ciliary arteries, two in number, pierce the 

 sclera on the medial and lateral sides of the optic nerve respectively, and extend 

 forwards, between the sclera and chorioid, towards the ciliary margin of the iris. 

 There each divides into a superior and an inferior branch, and the resulting four 

 branches anastomose in the form of a circle, termed the circulus arteriosus major. 

 This circle is joined by a varying number of anterior ciliary arteries, derived from 

 the lacrimal and muscular branches of the ophthalmic artery, and, after supplying 

 the ciliary muscle, sends converging branches towards the aperture of the pupil, 

 where a second circle, the circulus arteriosus minor, is formed. The veins proceed 



