304 



SEKOUS MEMBEANES. 



Fig. 96. 



Serous epithelium (en- 

 dothelium, His). Each 

 cell consisting of a 

 non-nucleated polygo- 

 nal plate and a nu- 

 cleated layer of proto- 

 plasm. The latter re- 

 tracted at a a. After 



Milncll. yrrrn. 



true one. The nucleus adheres to one surface, and always to 

 the under surface of the cell.* It is 

 attached hy means of a variable amount 

 of finely-granular protoplasm. Thus we 

 see that the cell, apparently so simple, 

 consists of at least two parts ; a homo- 

 geneous, glassy, polygonal plate, and a 

 nucleated mass of protoplasm. The quan- 

 tity of the latter is usually just enough to 

 fix the nucleus in its place under the middle 

 of the plate ; sometimes, however, there 

 may be enough of it to form a uniform 

 layer, conterminous with the homogeneous 

 plate. In the latter case the cells, when 

 looked at from above, exhibit a finely- 

 granular aspect. On turning our attention, 

 however, to the points at which the indi- 

 vidual cells are in contact, sre may often observe clear, circular 

 gaps in the granular layer, over which the homogeneous lamellas 

 extend uninterruptedly. These gaps are due to a slight reces- 

 sion of adjacent masses of protoplasm from one another 

 (fig. 96, a). 



The simplest explanation of this twofold structure of the 

 epithelial cells is afforded by the conception of the serous cavities 

 as interstices in the connective tissue. The epithelial cells which 

 line the serous membranes are not epithelial cells at all, in the 

 ordinary sense of the word ; they do not combine an indej^endent 

 morphological finality with an equally independent functional 

 completeness, like the epithelia of mucous membranes. The 

 epithelial cell of a serous sac is essentially an endothelial element. 

 It originates in a partial hardening of the protoplasm of a soft 

 connective-tissue corpuscle ; this portion forming the homo- 

 geneous plate, while the residual protoplasm together with the 

 nucleus, remain unaltered. This protoplasmic residue however, 

 belongs at once to the homogeneous plate, and to the inter- 



* This is unconditionally true only of the adult organism. In the 

 embryo, the homogeneous plate referred to in the text does not yet 

 exist, as a rule; and the future endothelial cells, T^-ith their abundant 

 protoplasm, often in a state of active proliferation, lie naked upon 

 the surface of the connective tissue. 



