EYE OF THE ENGLISH SPAEROW 407 



supplied with blood-vessels and lying between the retina and the 

 sclera, it furnishes these two layers with nourishment. No 

 blood-vessels are seen in the retina. This layer therefore must 

 be nourished by osmosis mainly from the chorioid. The chorioid 

 is thickest at the axis of vision and becomes gradually thinner 

 toward the front of the eye. At the axis it measures .134 mm. 

 and at the ora serrata .004 mm. in thickness. 



The blood for the chorioid is supplied by the short, the long, 

 and the anterior ciliary arteries. These are derived from the 

 ophthalmotemporal artery (fig. 15, p, c, C, and Ic). After pierc- 

 ing the sclera at several places they spread out and branch 

 profusely over the surface of the chorioid. The smaller branches 

 are soon hidden by the dense pigment. A group of thirteen 

 of these arteries is shown at C in figure 15. This region is 

 slightly anterior to the center of vision. The arrangement and 

 partial distribution of these arteries is shown in figure 16. All of 

 these branches lie in grooves on the surface of the chorioid until 

 they have reached their finer branches and disappear from view. 

 The entrance of most of these arteries at the posterior part of 

 the chorioid accounts for its much greater thickness in this 

 region. A cross-section of the chorioid shows the large blood- 

 vessels in the outer part of the layer and the small vessels 

 nearer the inner margin. The long ciliary arteries are derived 

 from a single artery which pierces the sclera a short distance 

 posterior to the entrance of the optic nerve. This single trunk 

 divides into a number of branches, some of which are distributed 

 over adjacent regions while others run forward on the surface of 

 the chorioid in grooves to the ciliary region. Other arteries, 

 the anterior ciliary, which are derived from the superior and 

 inferior ophthalmic arteries, occasionally pass through the 

 sclera and sclerotic bones to the chorioid and the ciliary muscles 

 (figs. 13, c, and 14). These branches and the long ciliary arteries 

 supply the ciliary processes, the muscles of accommodation, and 

 the iris. 



The nerve supply to the chorioid is derived from the short 

 and the long ciliary nerves (figs. 21 and 22). The short ciliary 

 nerves, after piercing the sclera in eight different places, divide 



