NORMAL HISTOLOGY. 



muscle and nerves. The stroma consists of loose spongy re- 

 ticular connective tissue greatly strengthened by the radially- 

 disposed blood-vessels and nerves, around which the delicate 

 stroma forms ensheathing masses of considerable density. The 

 clefts situated between these adventitious sheaths and the included 

 vessels and nerves form a system of lymphatic channels through- 

 out the iris which communicate with the anterior chamber through 

 the lymph-spaces at the irido-corneal angle. 



The arteries of the iris spring from the anterior part of the cir- 



culus arteriosus iridis major, 



FIG. 374. situated at the ciliary border, and 



pass towards the centre of the iris 

 as radially-disposed, freely-anasto- 

 mosing twigs ; about i mm. from 

 the inner edge of the iris these 

 vessels unite to form a second 

 delicate vascular ring, the circulus 

 arteriosus iridis minor, which 

 marks the division of the iris into 

 its pupillary and ciliary zones, 

 which are respectively i mm. and 

 3-4 mm. in breadth. From this 

 circle the arterioles continue their 

 course towards the pupillary bor- 

 der, and end in the capillary net- 

 work distributed to the sphincter 



muscle. Capillary reticula exist also within the anterior and pos- 

 terior layers of the stroma. All the capillaries are tributary to the 

 radiating veins which pass to the ciliary border, where they join those 

 of the ciliary processes and finally empty into the radicles forming 

 the venae vorticosse. 



Bundles of involuntary muscle occupy the pupillary border and 

 the posterior zone of the stroma-layer ; these are arranged as two 

 sets, the annular bundles encircling the pupillary margin of the iris 

 and constituting the sphincter of the pupil, a muscular zone about 

 i mm. in width, and the ill-developed radially-disposed bundles 

 extending from the pupil towards the ciliary margin and forming an 

 incomplete and very inconspicuous layer, the dilator pupillae. 



The posterior boundary layer, or vitreous lamella, appears 

 as a glassy structureless membrane, about 2 p. in thickness, which 

 stretches over the posterior surface of the stroma and supports 

 the pigment-layer : in the nature of its substance this structure 

 closely approaches elastic tissue. 



The pigment-layer, or pars iridica retinae, is usually so densely 



Injected iris from eye of dog : P, pupillary 

 margin, around which capillary net- work is 

 formed by vessels proceeding from lesser ar- 

 terial circle. 



