744 THE SENSE ORGANS AND SKIN OF THE HORSE 



but a ring of circular fibers forms the inner angle of its base. In the horse the muscle is much 

 less developed, and has the form of a fiat band; it does not contain circular fibers, but the arrange- 

 ment is rendered more or less plexiform by the existence of oblique and equatorial fibers. 



3. The iris (Figs. 552, 557, 559) is a musctilar diaphragm placed in front of the 

 lens, and is visible through the cornea. It is pierced centrally by an elliptical 

 opening, the pupil (Pupilla), which varies in size during life and determines the 

 amount of light admitted to act on the retina. In strong light the vertical diameter 

 of the pupil is very short, Init the opening is almost circular when the pupil is fully 

 dilated. The ciliary border (Margo ciliaris) or periphery of the iris is continuous 

 with the ciliary body and is connected with the corneo-scleral junction by the liga- 

 mentiun pectinatum iridis. The ligament consists of strands of connective tissue 

 which are attached externally to the corneo-scleral junction. Its bundles interlace 

 and inclose spaces (Spatia anguli iridis) which are lined with endothelium and com- 

 municate with the anterior chamber. The pupillary border (Margo pupillaris) 

 surrounds the pupil. Its upper part bears in its middle several black masses of 

 variable size, termed the granula iridis or corpora nigra; similar, but much smaller, 

 projections may be seen on the lower margin of the pupil. The anterior surface 

 (Facies anterior) determines the color of the eye, which is dark brown usually. It 

 is marked by delicate concentric lines (Plicae iridis), which fade out near the pupil. 

 The smooth, narrow, central part is termed the pupillary zone or annulus iridis 

 minor, while the mtich broader plicated part is the ciliary zone or annulus iridis 

 major. The posterior surface (Facies posterior) is usually black. It bears 

 numerous fine radial lines except at the pupillary margin. Its central part is in 

 contact with the anterior surface of the lens, but peripherally the two are separated 

 by a narrow space termed the posterior chamber. The iris consists chiefly of the 

 stroma iridis, a delicate framework of connective tissue, which supports numerous 

 blood-vessels, and contains branched pigmented cells. The muscular tissue 

 (unstriped) consists of a sphincter and a dilator of the pupil. The sphincter 

 pupillae lies in the posterior part arotmd the pupil, with which the fibers are largely 

 concentric. The dilatator pupillae consists of fillers which radiate outward from 

 the sphincter to the ciliary l)order. The anterior surface of the iris is covered iDy a 

 continuation of the endothelium of the cornea. Beneath this is a condensation of 

 the stroma, in which the cells are close together and are full of pigmented granules.^ 

 There appear to be minute clefts here by which the lymph-spaces of the stroma 

 commtmicate with the anterior chamber. The posterior stu'face is covered by a 

 pigmented epithelium, the stratum pigmenti iridis, which is a continuation of that 

 of the ciliary body. 



The arteries of the vascular tunic come from the ciliary branches of the oph- 

 thalmic artery. The arteries of the chorioidea are derived chiefly from the short 

 posterior ciliary arteries. These perforate the sclera around the posterior pole, 

 run forward in the lamina vasculosa, and form the rich capillary network of the 

 choriocapillaris. The two long ciliary arteries perforate the sclera obliquely near 

 the optic nerve; they run forward in the lamina suprachorioidea in the horizontal 

 meridian, one on the inner, the other on the outer, side of the eyeball. On reaching 

 the ciliary body each divides into diverging ])ranches; the subdivisions of these 

 unite with each other and with twigs of the anterior ciliary arteries to form a circular 

 anastomosis, the circulus arteriosus major. From this branches go to the ciliary 

 muscle and processes and to the iris. The two anterior ciliary arteries, dorsal and 

 ventral, form an episcleral plexus aroimd the corneo-scleral junction, and give off 

 branches which perforate the sclera. These supply twigs to the ciliary muscle and 

 recurrent branches to the chorioid, and assist in forming the circulus arteriosus 

 major. 



The blood is carried away from the vascular tunic chiefly by four or five venous 



^ In albinos the pigment is absent here, as elsewhere, and the iris is pinkish in color. 



