340 MANUAL OF HISTOLOGY. 



nection between the short posterior or choroidal arteries pro- 

 per and those which supply the circulus arteriosus. A small 

 amount of the blood which returns from the capillaries of the 

 choroid, ciliary body and iris finds its exit from the eyeball 

 through the veins accompanying the anterior and posterior 

 ciliary arteries, but by far the larger part is collected by the 

 large veins in the outer layers of the choroid (venae vorticosse), 

 converging so as to form four or six great trunks, which perfo- 

 rate the sclera obliquely in the equatorial region, and empty 

 into the ophthalmic vein. 



The long and short ciliary nerves supply the tunica vascu- 

 losa with fibres from the third and fifth pair and the sympa- 

 thetic. The long nerves, two or three in number, are branches 

 of the nasal division of the ophthalmic nerve ; the short, ten to 

 fifteen in number, arise from the ciliary ganglion. These nerves 

 penetrate the sclera near the optic nerve, and then, passing for- 

 ward on the outer portion of the choroid, form, in the ciliary 

 muscle, a fine plexus with ganglionic corpuscles at the nodal 

 points of the meshes ; from this plexus fibres are distributed 

 to the cornea and iris. 



At the junction of the anterior and middle thirds of the 

 eyeball the choroid undergoes a change, the membrane be- 

 comes thinner, the capillaries turn back toward the veins, only 

 a few vessels continuing forward in a straight course. 



In this region the retina also undergoes a change and loses 

 all its nervous elements, the connective tissue supporting fibres 

 alone being continued forward under the name of the pars cili- 

 aris retinae. The very narrow zone between the points where 

 these changes occur and the irregular line formed by the begin- 

 ning of the ciliary processes is called the orbiculus ciliaris^ 

 and the line of origin of these processes the ora serrata. 



The ciliary body. Crossing the orbiculus, the choroid is 

 seen raised in radial folds, some seventy in number, which in- 

 crease in size until they reach the thickness of a millimetre. 

 This increase is caused by the development of smooth muscular 

 fibres in addition to the usual constituents of the choroid. 



These fibres arise just behind the canal of Schlemm, from 

 the sclera and cornea ; passing backward, they together form 

 a ring, which on section appears as a right-angled triangle, 

 with the base turned toward the anterior chamber, and the hy- 

 pothenuse toward the vitreous (Fig. 155). 



