EPITHELIAL AND ENDOTHELIAL TISSUE. 331 



atory, manufacturing chemical products from the plasma of the 

 blood, and probably also from the red blood-corpuscles. The col- 

 oring matter of the bile is known to be closely allied to haemo- 

 globine. In the fluid of the semen formations of living matter 

 are suspended the spermatozoids, which are direct offsprings of 

 the epithelia of the testicles. Saliva represents an intermediate 

 condition between the watery and mucous secretions. 



R. Heidenhain first drew attention to the differences in the 

 aspect of the epithelia of the salivary glands, under the different 

 conditions of fullness and contraction. He found the epithelia 

 in starved dogs light colored, very much swelled, partly destitute 

 of granules, and unaffected by the carmine stain, although the 

 nucleus would be deeply colored. In epithelia of dogs, which 

 before death were abundantly fed, or whose salivary glands, by 

 means of electricity, were stimulated to an intense and long- 

 continued secretion, the epithelia were, on the contrary, small, 

 regular in outline, and their nuclei less stained with carmine. 

 In the first case, the central caliber was imperceptible j in the 

 latter, it was very distinct. The same observer discovered in the 

 epithelia of the pancreas, also, peculiarities depending upon the 

 state of the secretion. He called the upper portion of the epi- 

 thelium granular, the lower light portion structureless ; and in 

 the process of secretion the upper layer predominated largely. 

 Heidenhain and Rollett discovered varieties of epithelia in the 

 pepsine glands, which were unlike in appearance, some being 

 very large and swelled, with a distinct nucleus; others small, 

 about the size of the ordinary cuboidal epithelia, and not dis- 

 tinctly nucleated. These facts, inexplicable to their discoverers 

 and more recent investigators, are fully understood by recog- 

 nizing the presence of the bioplasson reticulum of the epithelial 

 bodies within the investing layer of cement-substance. When 

 the epithelia of the salivary, the pancreatic, and the stomachic 

 glands are laden with secretion, the meshes of the reticulum 

 are enlarged, and the reticulum itself stretched ; but when the 

 epithelia are emptied of their secretion, the reticulum is in a con- 

 dition of equilibrium. Bioplasson is accumulated in larger 

 quantity at the portion of the epithelia, nearest to the connective 

 tissue, exhibiting at all times a nearly homogeneous, shining 

 appearance. 



Unquestionably, in the process of mucous secretion a portion, 

 or it may be the whole, of the bioplasson of individual epi- 

 thelia is destroyed. With this fact before us we can readily 



