604 A TEXTBOOK OF PHYSIOLOGY 



Functions of the Ciliary Body. The ciliary muscle effects the ac- 

 commodation of the eye (see later, p. 614); the ciliary processes 

 secrete the aqueous humour, the pigment in the ciliary bodies 

 assists the iris in preventing light passing through the periphery of 

 the lens. 



The Iris is the pigmented curtain or diaphragm which lies in the 

 aqueous chamber, dividing it into an anterior chamber in front and 

 a posterior chamber behind. The aperture in the centre forms the 

 pupil of the eye. At its base, where it is thinnest, the iris is con- 

 tinuous with the anterior part of the ciliary body, both as regards 

 stroma and the pigmented and epithelial cells, which lie on its posterior 

 surface. The epithelial cells on the front of the iris are reflected at 

 the corneo-iridic angle on to the back of the cornea. Both sets of 

 epithelial cells, anterior and posterior, lie on basement membranes, 

 in between which there is a delicate network of connective tissue 

 containing the pigment cells which give the iris its characteristic v 

 colour. This connective tissue contains a network of capillaries 

 which run into a large vein at the base the circulus iridis major. 

 It also forms lymph spaces, which connect by minute crypts at the 

 base of the iris with the aqueous fluid in the anterior chamber. By 

 these crypts, therefore, aqueous fluid can pass into the lymph spaces 

 of the iris, and thence into the iris veins. 



In this connective tissue, also, lie the muscle fibres of the iris. 

 These are of the smooth variety, and consist of two sets: (1) Circular 

 fibres, developed mostly near the free pupillary margin, and sphincter 

 in action the sphincter pupillse ; (2) meridional fibres arranged about 

 the radial furrows of the iris. These fibres arise from the innermost 

 strands of the cribriform ligament, and terminate in the connective 

 tissue of the sphincter pupillae. 



The sensory nerve of the iris is the nasal branch of the ophthalmic 

 division of the fifth nerve. The motor nerve to the sphincter muscle 

 is the inferior division of the third nerve. This has its cell-station 

 in the ciliary ganglion, and passes to the iris as the short ciliary nerves. 

 The nerve to the dilator pupillse is the cervical sympathetic. Its 

 cell-station is in the superior cervical ganglion, and the fibres reach 

 the eye through the ophthalmic division of the fifth and the long 

 ciliary nerves. 



The Function of the Iris. The function of the iris is to regulate 

 the amount of light admitted to the eye, and to diminish spherical 

 and chromatic aberration (see later, p. 609). Its action is reflex. 

 It contracts when strong light falls on the retina (the light reflex). 

 When light falls on one eye, the pupil of the other eye also 

 contracts (the consensual light reflex). The iris also contracts during 

 accommodation of the eye for a near object, during sleep, and under 

 the action of certain drugs known as myotics e.g., morphia, which 

 acts centrally, and pilocarpine, eserine, etc., which act locally. Besides 

 stimulation of the third nerve, section or paralysis of the cervical 

 sympathetic nerve will cause contraction of the pupil. Dilatation of 



