30 



THE EYE. 



the choroidal membrane, Several small arterial branches enter the outer part of each 

 ciliary process, at first running parallel to each other and communicating sparingly. 

 As they enter the prominent folded portion, the vessels become tortuous, subdivide 

 minutely, and inosculate frequently, Finally they form short arches or loops, and 

 turn backwards to pour their contents into the radicles of the veins. On the free 

 border of the fold, one artery, larger than the rest, extends the whole length of each 

 ciliary process, and communicates through intervening vessels with a long venous 

 trunk which runs a similar course on the attached surface. 



Ciliary muscle. The anterior part of the choroid, opposite the ciliary pro- 

 cesses, is considerably thickened. The thickening is triangular in section, and is 

 produced by a zone of plain muscular tissue, forming the ciliary muscle (Bowman) 

 (tensor choroidecz, Bruecke). The main part of this muscle arises (figs. 35, 36) by 

 a thin flat expansion from the fore part of the sclerotic close to the cornea, between 



A Fig. 37. DIAGRAMS SHOWING THK 



COMPARATIVE: DEVELOPMENT OF 



THE CIRCULAR FIBRES OF THE 

 CILIARY MUSCLE IN 



A, the normal or emraetropic eye ; 



B, the hypermetropic eye ; and 



C, the myopic eye. (Fuchs. ) 



the canal of Schlemm and the 

 anterior chamber, its fibres 

 being attached to the bundles 

 of the ligamentum pectinatum 

 which occupies this position. 

 The muscular fibres, spread- 

 ing radially (meridionally), 

 are directed backwards (fig. 

 35, M), to be inserted into 

 the choroid coat opposite to 

 the ciliary processes, and 

 partly further back. At their 

 insertion the fibres pass 

 obliquely and inter-cross so 

 as to form peculiar stellate 

 figures. According to Wal- 

 deyer, a small portion (the 

 outermost) is sometimes in- 

 serted into the sclerotic coat, 1 

 but this must be very rare, 

 for the sclerotic always be- 

 comes detached with the 



greatest ease from this part of the muscle. The radial fibres pass towards the iris 

 into a ring of fibres (fig. 35, Mu\ which have a circular course around the insertion 

 of the iris. This set forms the circular ciliary muscle of H. Miiller and is much 

 developed in hypermetropic eyes, but atrophied, or even absent altogether, in myopic 

 (Iwanoff) (fig. 37). It was formerly described as the ciliary ligament. In birds the 

 ciliary muscle is composed of cross-striped muscular fibres. When the ciliary muscle 

 contracts it tends to draw the choroid and ciliary processes forwards and inwards. 



1 This part, if present, is the homologue of an important portion of the ciliary muscle of birds 

 which is known as the Cramptonian muscle. 



