VI 



DIOPTKIC MECHANISM OF THE EYE 



269 



is readily detached in macerated eyes (Fig. 107). On this elastic 

 membrane known as the -membrana basilaris or membrane of 

 Bruch rest the hexagonal pigment cells, which form the tapetum 

 nigrum and are in close morphological relation with the retina ; 

 these were for a long time erroneously held to belong to the internal 

 layer of the choroid. 



The ciliary portion of the uvea or zona ciliaris extends from 

 the ora serrata to the pars iridica retinae. In the ciliary body 

 the uvea becomes much thicker than in the posterior part of the 

 choroid, owing to the appearance of two new formations, the 

 ciliary muscle externally, and the corona of the ciliary processes 

 internally. 



The ciliary muscle is triangular in section. Owing to the 

 direction of the smooth muscle-fibres of which it is made up it 

 is usually regarded as two distinct muscles, an outer, consisting 

 of fibres running in a' meridional direction (Bruch's muscle, or 



** ---". .^ 



FIG. 107. Section of choroid coat. (Cadiat.) a, membrana basilaris or membrane of Bruch ; 

 immediately above is the lamina chorio-capillaris ; b, lamina vasculosa ; c, vein with blood 

 corpuscles ; d, lamina supra-choroidea. 



musculus tensor choroidae), and an inner, consisting of fibres 

 forming a ring or sphincter round the insertion of the iris 

 (Miiller's muscle, or the circular ciliary muscle). Fig. 108 shows 

 the fibres of the former in longitudinal, of the latter in transverse 

 section. A third group of muscle-fibres run obliquely, interlacing 

 so as to form a kind of network, their direction being intermediate 

 between that of Bruch's and of Miiller's muscles. 



The ciliary processes, about seventy in number, form a circle 

 of radial thickenings, which project into the anterior part of the 

 vitreous humour (Fig. 109). They are free from the pigment 

 which invests the remainder of the ciliary body, and -contain a 

 rich plexus of vessels, which anastomose and divide frequently. 

 Between each two well-developed ciliary processes there are either 

 smaller processes or an arteriole which runs direct to the iris. 



The iris is a perforated membranous diaphragm placed in 

 front of the crystalline lens, slightly convex towards the edge of 

 the pupil and concave towards the periphery. The aperture of 

 the pupil is not quite in the centre, but slightly inward. The 



