20 



NORMAL HISTOLOGY. 



FIG. 24. Pigmented epi- 

 thelium from the human 

 retina. X 435- 



spherical and polyhedral, in the one case, and upon the number and nature 

 of the secretion particles in the other. The cells lining parts of certain 

 glands, as those clothing the ducts of the salivary glands, or the convoluted 

 portion of the uriniferous tubules, exhibit a more or 

 less evident striation. Such cells constitute rod epi- 

 thelium. 



The highest and most complex modifications of 

 epithelial tissues are those occurring during the devel- 

 opment of the structures designed to receive the stimuli 

 giving rise to the special senses. The epithelium in 

 these localities is differentiated into two groups of 

 elements, the sustentacular and the perceptive; to the 

 latter the name of neuro- epithelium is applied. Con- 

 spicuous examples of such specialized epithelium are 

 the rod- and cone-cells of the retina and the auditory 

 or hair-cells of Corti's organ in the internal ear. 

 A more detailed description of glandular epithelium is given in the 

 chapter devoted to Mucous Membranes and Glands (page 119); the details 

 of the neuro-epithelial structures are included under the appropriate Organs 

 of the Senses. 



THE ENDOTHELIA. 



This term, as here used, is applied to the modified mesodermic cells 

 that cover serous surfaces and, therefore, includes the lining of the pericardial, 

 pleural and peritoneal subdivisions of the body-cavity, together with the 

 lining of the blood- and lymph-vessels and of the lymphatic spaces through- 

 out the body. In principle, these spaces are intramesodermic clefts and the 

 elements forming their lining are derivatives of the great connective tissue- 

 producing germ-layer. The endothelia, therefore, are closely related to the 

 connective tissues and, in a sense, may be regarded as modified elements of 

 that class. In view of their arrangement as investing cell-sheets and other 

 resemblances, they may be 

 conveniently discussed in con- 

 nection with the epithelial 

 tissues; indeed by many his- 

 tologists they are included 

 among the epithelia. 



The most striking differ- 

 ence in situation between the 

 endothelia and the epithelia is 

 the fact of the former covering 

 surfaces not communicating 

 with the atmosphere, while the 

 epithelial tissues clothe mu- 

 cous membranes, all of which 

 are directly or indirectly con- 

 tinuous with the integumen- 

 tary surface. A further con- 

 trast between these tissues is 

 presented in their genetic 

 relations with the germ-layers, since the epithelia, with the exception of those 

 lining certain parts of the genito-urinary tracts which are derived from the 

 mesoderm, are transformations and outgrowths from the ectoderm and ento- 



FlG. 25. Mesothelial cells from surface of pmentutn; intercel. 

 lular cement-substance stained by argentic nitrate. X 300. 



