544 



THE NERVOUS SYSTEM. 



varying in number from twenty-four to forty-five, according to 

 different authorities. These nerves terminate in the subepithelial 

 plexus, beneath the superficial epithelium, from which fibers pass 

 to the cells themselves, forming the intra-epithelial plexus. 



Choroid. This vascular and pigmented structure forms the 

 posterior five-sixths of the second tunic of the eye. Its anterior 

 boundary is the ciliary ligament formerly so called, but also known 

 as the ring muscle of Muller ; it is composed of the circular fibers 

 of the ciliary muscle. Anterior to this is the iris. At the anterior 

 margin of the choroid are the ciliary processes. The inner sur- 

 face of the choroid is in contact with the retina. 



The choroid is composed of three layers : (1) lamina supra- 

 choroidea (Fig. 322), the most external, consisting of connective 

 tissue, elastic fibers, pigment-cells, and lymph-corpuscles. It is 



Corneal epithe- 

 1 Hum. 



Substantia pro- 

 pria. 



Descemet's 



membrane. 

 Canal of 



Schlemm. 

 Iris. 

 ; Pigment-layer. 



- Meridional fibers. 



- Radial fibers. 

 2 Muller's fibers. 



Conjunc 

 tiva. 



Sclera. 



Processus ciliares. 



FIG. 323. Meridional section of the human ciliary body; X20 (Bohm and 



Davidoff). 



in contact with the lamina fusca of the sclerotic. (2) Vascular 

 layer; this is spoken of as the ch,oroid proper; here are the 

 blood-vessels of the choroid, consisting externally of branches of 

 the short ciliary arteries, and veins arranged in a vorticose, 

 whorled, or star-like form, constituting the venae vorticosce, 

 which converge to form four or five main veins; between 

 these vessels are stellate pigment-cells. The inner portion of the 

 vascular layer is the choriocapillaris or tunica Ruyschiana, a plexus 

 of fine capillaries from the short ciliary vessels. (3) The most 

 internal portion of the choroid is the lamina vitrea, glassy layer, or 

 membrane of Bruch, a thin membrane, transparent, and ordinarily 

 described as structureless, although Kolliker considers it to have a 

 fibrous structure. This is in contact with the pigmentary layer 

 of the retina. 



The arteries of the choroid are the short ciliary and the recur- 



