THE EYE. 



249 



(Leber). Others regard it as a lymphatic passage (Schwalbe, 

 Waldeyer). 



Posteriorly, the sclerotica passes over into the external 

 sheath of the optic nerve, derived from the dura mater. This 

 membrane is finally strengthened by the insertions of the ten- 

 dinous bundles of the ocular muscles. 



The system of the uvea, with the exception of its most 

 anterior portion, the iris, is characterized by very considerably 

 developed vessels. 



The entire inner surface (and the posterior surface of the 

 iris) is covered by the pigmented outer epithelium of the 

 retina (p. 30). During a portion of the fcetal period, the 

 latter extended much further forwards than it does at a more 

 mature period. 



The greater portion of the uvea is formed by the posterior 

 segment, the choroid. The thin membrane consists of sev- 

 eral, not sharply demarcated, connective-tissue layers. 



We recognize a, an inner hyaline boundary layer, 0.0006 to 

 O.OO08 mm. in thickness, thicker and more uneven in front ; b, 

 a thin homogeneous layer, with extraordinarily developed 

 stellate capillary net-works (choroidea >capillaris) ; c, the 

 choroid proper of the histologists, with stellate, very generally 

 pigmented connective-tissue cells, and a great wealth of arte- 

 rial, as well as venous vessels ; and, finally, d, a loose pig- 

 mented connective tissue, which forms the connection with 

 the inner surface of the sclerotica. It is called the lamina 

 fusca, and also the supra-choroidea ; it forms a lymphatic 

 space. 



The vascular net-work in the ciliary body, and in the ciliary 

 processes which project inwards from the latter, is greatly 

 developed. The substratum remains similar to that of the 

 choroid, though the pigmented connective-tissue cells dis- 

 appear. 



Externally to these processes we meet with a peculiar 

 smooth muscular mass, the tensor choroideae, musculus ciliaris, 

 or ligamentum ciliare of an older epoch (Fig. 199, /). 



The human ciliary muscle arises from the inner side of the 

 11* 



