SECKETION. 



319 



ment-membrane; certain cells; and blood-vessels. These three structural 

 elements are arranged together in various ways; but all the varieties nw^ 

 be classed under one or other of two principal divisions, namely, mem- 

 branes and glands. 



ORGANS AND TISSUES OF SECRETION. 



The principal secreting membranes are (1) the Serous and Synovial 

 membranes; (2) the Mucous membranes; (3) the Mammary gland; (4) 

 the Lachrymal gland; and (5) the Skin. 



(1) Serous Membranes. The serous membranes are especially dis- 

 tinguished by the characters of the endothelium covering their free sur- 





FIG. 219. Section of synovial membrane, a, endothelial covering of elevations of the membrane ; 

 fo, subserous tissue containing fat and blood-vessels; c, ligament covered by the synovial membrane. 



face: it always consists of a single layer of polygonal cells. The ground 

 substance of most serous membranes consists of connective-tissue cor- 

 puscles of various forms lying in the branching spaces which constitute 

 the ' 'lymph canalicular system" (p. 292), and interwoven with bundles 

 of white fibrous tissue, and numerous delicate elastic fibrillae, together 

 with blood-vessels, nerves, and lymphatics. In relation to the process of 

 secretion, the layer of connective tissue serves as a groundwork for the 

 ramification of blood-vessels, lymphatics, and nerves. But in its usual 

 form it is absent in some instances, as in the arachnoid covering the 

 dura mater, and in the interior of the ventricles of the brain. The 

 primary membrane and epithelium are always present, and are concerned 



