346 



NORMAL HISTOLOGY. 



Pupillary border, 

 of iris 



Arterial circle 1 



Artery 



Ciliary processes 



The sphincter muscle is a band of involuntary tissue about .7 mm. In 

 width, surrounding the pupil and situated in the vascular stroma, back of the 

 blood-vessels. 



The dilatator muscle is a sheet of smooth muscle-fibres in the position 

 formerly described as the posterior limiting lamella, or membrane of Bruch. 



Investigations have settled 

 definitely the question of its 

 existence and shown that its 

 fibres arise from the outer 

 cells of the retinal pigment 

 layer, on the posterior sur- 

 face of the iris. They are 

 best developed toward the 

 periphery of the iris. 



The posterior surface 

 of the iris is covered by the 

 pigmented layer, which 

 morphologically represents 

 the anterior segment of the 

 atrophic nervous tunic {pars 

 iridica retina). This is con- 

 tinuous with the pigmentary 

 layer covering the ciliary 

 processes, but the cells, dis- 

 posed as a double layer, are 

 so deeply pigmented as to 

 be indistinguishable without 

 bleaching the tissue. Since 

 the iridial muscle is de- 

 veloped from the fusiform 

 cells of the outer layer it rep- 

 resents an epithelial (ecto- 

 dermic) muscle. The inner 

 cells are larger polygonal elements, which gradually lose their pigment as 

 they approach the ciliary processes. Over the latter they contain no pig- 

 ment, whereas the outer cells remain pigmented. 



The blood-vessels of the iris pass radiallyinwards from the circulus arteri- 

 osus iridis major at the periphery. Near the pupillary border, they form a 

 second ring, the circulus arteriosus iridis minor, branches from which 

 supply the sphincter muscle and the pupillary zone. The venous radicles 

 unite to form trunks which accompany those from the ciliary processes to 

 mpty into the vence vorticosce. The lymphatics are represented by the 

 interfascicular clefts which communicate with the anterior chamber, with the 

 spaces within the ciliary body, and with the spaces of Fontana. The nerves 

 of the iris, branches of the ciliary nerves, follow the course of the blood- 

 vessels and, branching, form a plexus of nonmedullated fibres, which supply 

 chiefly the involuntary muscle, including that of the vessels. 



Choroidal 

 veins 



FIG. 395. Injected ciliary processes and iris; posterior surface. 

 X 20. 



THE NERVOUS TUNIC. 



The Retina. The retina, the light-perceiving portion of the eye, 

 represents a modified portion of the brain itself, with which it develops in 

 close connection. It is a delicate membrane and extends from the optic 



