684 ORGANS OF SENSE. 



Externally, it is connected by a fine cellular web (membrana fusca) with the 

 inner surface of the sclerotic. Its inner surface is smooth, and lies in contact 

 The choroid is composed of three layers, 



Fig. 370. The Veins of the Choroid. (Enlarged.) 



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with the retina. The choroid is composed of three layers, external, middle, 

 and internal. 



The external layer consists, in part, of the larger branches of the short ciliary 

 arteries, which run forwards between the veins before they bend downwards to 

 terminate on the inner surface. This coat is formed, however, principally of 

 veins, which are named, from their distribution, vense vorticosse. They converge 

 to four or five equidistant trunks, which pierce the sclerotic midway between 

 the margin of the cornea and the entrance of the optic nerve. Interspersed 

 between the vessels are lodged dark star-shaped pigment-cells, the fibrous offsets 

 from which, communicating with similar branchings from neighboring cells, 

 form a delicate network, which, towards the inner surface of the choroid, loses 

 its pigmentary character. 



The middle layer consists of an exceedingly fine capillary plexus, formed by 

 the short ciliary vessels, and is known as the tunica Ruyschiana. The network 

 is close, and finer at the hinder part of the choroid than in front. About half 

 an inch behind the cornea, its meshes become larger, and are continuous with 

 those of the ciliary process. 



The internal, or pigmentary layer, is a delicate membrane, consisting of a 

 single layer of hexagonal nucleated cells, loaded with pigment-granules, and 

 applied to each other, so as to resemble a tessellated pavement. Each cell con- 

 tains a nucleus, and is filled with grains of pigment, which are in greater 

 abundance at the circumference of the cell. In perfect albinos this epithelium 

 contains no pigment, and none is present in the star-shaped cells found in the 

 other layers of the choroid. 



The ciliary processes should be next examined : they may be exposed, either by detaching the 

 iris from its connection with the ciliary ligament, or by making a transverse section of the 

 globe, and examining them from behind. 



The Ciliary processes are formed by the plaiting or folding inwards of the 

 middle and internal layers of the choroid, at its anterior margin, and are received 

 between corresponding foldings of the suspensory ligament of the lens, thus 

 establishing a communication between the choroid and inner tunic of the eye. 

 They are arranged in a circle, behind the iris, round the margin of the lens. 

 They vary between sixty and eighty in number, lie side by side, and may be 

 divided into large and small ; the latter, consisting of about one-third of the 

 entire number, are situated in the spaces between the former, but without regu- 



