ORGANS OF THE BODY. 



621 



Fig. 578. Polyhedral pigment cells 

 from the choroid of the sheep, a, 

 mosaic of six-sided cells; 6, a 

 larger octagonal example. 



Apart from this cellular coating, the uvea consists of several layers, not 

 in all cases, however, distinctly defined one against the other. 



First, and most internally, we find a transparent limiting layer, smooth, 

 and only O'OOOG-O'OOOS mm. in the fundus of the eye-ball, but thicker, 

 and presenting a more uneven internal surface anteriorly. 



The next stratum is the choriocapillaris, an extremely dense network 

 of nucleated capillaries (already spoken of in 311), imbedded in a simple 

 connective-tissue. This stratum extends to the ora serrata. 



The third layer, the proper choroid, consists of a network of ramifying 

 stellate, or irregularly jagged connective-tissue cells, with thread-like pro- 

 cesses of varying length. These cells are remarkable for the great avidity 

 with which they take up dark black pigmentary matters (fig. 579). 

 We have already considered them under the name of "stellate pigment 

 cells" in speaking of connective- tissue, p. 219. But what particularly 

 characterises this layer further is the great abundance of arterial and 

 venous vessels found in it. The first present a strongly developed mus- 

 cular tunic. Longitudinal bundles of invo- 

 luntary muscular fibres also occur in the pos- 

 terior segment of the choroid accompanying 

 these arterial vessels (H. Mutter), and lymph 

 cells likewise (G-. Haase). 



Externally the choroidal tissue is con- 

 tinuous, in the form of a soft, brownish con- 

 nective substance with the sclerotic, and is 

 known here as the lamina fasca or supra- 

 choroidea. 



Anteriorly, as is well known, the choroid is continuous with the 

 corpus ciliare and its numerous ciliary processes projecting inwards. 

 These structures are likewise clothed with the same flattened pigmentary 

 epithelium. Here, however, the latter has become laminated, or at least 

 double-layered (p. 143). 



On and within the corpus ciliare, whose tissue resembles that of the 

 choroid (although pigmentary cells are 

 fewer in it), and which also possesses the 

 same delicate limiting membrane as the 

 latter, a peculiar involuntary muscle pre- 

 sents itself, the tensor cltoroidea, or 

 musculis ciliaris (fig. 573, /), for the dis- 

 covery of which we are indebted to Brucke 

 and Bowman, while many important 

 points, in regard to its nature, were sub- 

 sequently put forward by H. Midler. 

 This muscle, which has been so frequently 

 the subject of investigation, was formerly 

 held to be nothing but simple connective- 

 tissue, under the name of the ligamentum ciliare. 



It springs (fig. 580), at the line of union between the cornea and sclerotic 

 coat, from the fibrous tissue which forms the internal wall of the canal of 

 Schlemm, and its distribution is to the tissue of the ciliary body. Its 

 fasciculi, closely crowded, maintain from this point of origin a radiating 

 or meridional course backwards, and are lost in the external portion of 

 the corpus ciliare. Here it is separated from the sclera by a thin prolon- 

 gation of the so-called suprachoroidea (Henle, Schulfze). Internally, that 



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Fig. 579. Pigmentary connective-tissue 

 corpuscles (so-called stellate pigment 

 cells), from the lamina fusca of a mam- 

 malian eye. 



