398 



HISTOLOGY 



part, in that the secretion appears as little globules, or in vacuoles, in 

 the proximal end of the cell and either moves through or with the cyto- 

 plasm toward the distal end, where it ruptures its bounds and is set free 

 in the lumen, together with a disintegrating portion of the cytoplasm. 

 Notwithstanding this probable cytoplasmic movement, the single nucleus 

 remains in its proximal position in the cell. 



The secretion stains black in osmic acid, but is not a real fat because 

 it dissolves in water. 



Another form of serous lubrication takes place in the cavity between 

 two bones that form a joint in the vertebrate animals. The membrane 

 which closes in this joint cavity at the sides is known as a synovial mem- 

 brane, and through its agency is produced the synovial fluid. 



The real secretory cells that produce the synovial fluid are not clearly 

 defined from the connective tissues that make up the bulk of this mem- 

 brane. In larger joints, 

 the membrane is evagi- 

 nated into the cavity, as 

 a series of short papillae 

 in some forms, or as a 

 single, septum-like lamella 

 which reaches as far as 

 the danger of being 

 pinched between the 

 bones will permit. The 

 synovial membrane of 

 the cat is amplified in 

 this latter fashion, and 

 the illustration, Figure 

 362, shows a vertical 

 section through a portion 

 of this lamella near its 

 inner boundary. 



The connective tissue 

 that forms the central part of this organ is arranged as a rather 

 wide-meshed reticulum. Blood vessels and lymphatics pass through 

 this reticulum, and from them is derived the water and the very small 

 proportion of other matter which makes up the synovial fluid. The 

 synovial cells that secrete the fluid must exert some specific influ- 

 ence on the selection of its proper constituents. They possess a 

 cytoplasmic body of some size and solidity, and their position on the 

 surface gives them an epithelial arrangement. 



The sweat glands of some mammals are perhaps to be considered 

 here. While they perform no real function of mechanical lubrication, 



bl. \M. 



FIG. 362 . Part of a vertical section through the synovial 

 lamella from a cat's joint, bl.ca., blood capillary con- 

 taining a corpuscle. 



