THE VASCULAR TUNIC. 



343 



The choroid consists of four layers, which from without inwards, are: 

 (i) the lamina suprachorioidea, (2) the choroid proper ; which contains the 

 larger vessels, (3) the choriocapillaris, or layer of fine capillaries, and (4) 

 the membrana vitrea. 



The lamina suprachorioidea is the outer boundary of the choroid and 

 connects it with the sclera. It is composed of interlacing bundles of fibrous 

 tissue, which are strengthened by rich networks of elastic fibres. The cellular 

 elements consist of (a) flattened endothelial plates, which line the lymph- 

 clefts and cover the connective tissue trabeculae connecting the choroid and 

 the sclera ; and (6~) large irregularly branched connective tissue cells, the 

 chromatophores, which are conspicuous on account of their deep pigmentation. 

 The lamellae of the suprachoroid continue, without definite boundary, into 

 the subjacent choroidal stroma. 



The choroid proper, as the choroidal stroma is called, has the same 

 general structure as the suprachoroidal layer, but the connective tissue 

 elements are denser and support a large number of blood-vessels, between 

 which are placed the stellate chromatophores. The largest vessels occupy 

 the outer part of the coat and are chiefly venous. They are surrounded 

 with perivascular lymph-sheaths, and converge in peculiar whorls to form 

 four or five large trunks, the ven<z vorticosa, which pierce the sclera in the 

 equatorial region and drain not only the choroid, but partly also the ciliary 

 body and iris. The arteries, derived from the short ciliary vessels, lie internal 

 to the veins. Their walls contain longi- 

 tudinally disposed muscle-fibres in addi- 

 tion to the customary circular ones. 



The choriocapillaris, or membrane 

 ofRuysch, is composed of capillaries which 

 form an extremely close meshwork em- 

 bedded within a homogeneous nonpig- 

 mented matrix. Between the choriocap- 

 illaris and th$ layer of larger vessels is a 

 narrow boundary zone of closely woven 

 fibro-elastic strands, which is nearly free 

 from pigment. In some animals this layer 

 possesses a peculiar metallic reflex and is 

 known as the tapeiumfibrosum; in carniv- 

 ora its iridescent appearance is due to the 

 presence of cells containing minute crystals 

 (tapetum. cellulosum). 



The membrana vitrea, or mem- 

 brane of Bruch, the innermost layer of the 

 choroid, measures only 2 n in thickness. 

 It separates the choriocapillaris from the retina and consists of two strata, 

 an inner homogeneous one, probably a product of the retinal pigment cells, 

 and an outer highly elastic portion. The nerves of the choroid form a 

 plexus within the lamina suprachorioidea, which contains groups of ganglion- 

 cells and sends numerous nonmedullated fibres chiefly to the muscular coats 

 of the arteries. The choroid contains no sensory nerve-fibres. 



The Ciliary Body. The ciliary body, the middle portion of the vas- 

 cular tunic, extends from the ora serrata to the sclero-corneal junction. 

 Sections through the eyeball in a meridional direction (Fig. 392) show its 

 triangular form. The outer side is in apposition to the sclera, the inner is 

 covered by the pigmented extension of the retina, and the short anterior 



FIG. 391. Portion of injected choroid, show- 

 uriace view of choriocapillaris layer. 



ing 

 X 130. 



