326 HANDBOOK OF PHYSIOLOGY. 



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

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

 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 Serons 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 distinguished by the characters 

 of the endothelium covering their free surface: it always consists of a 

 single layer of polygonal cells. The ground substance of most serous 

 membranes consists of connective-tissue corpuscles of various forms lying 

 in the branching spaces which constitute the " lymph canalicular sys- 

 tem " (p. 299), 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 connec- 

 tive tissue serves as a ground-work for the ramification of blood-vessels, 

 lymphatics, and nerves. But in its usual form it is absent in some in- 

 stances, 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 in the formation of the fluid by 

 which the free surface of the membrane is moistened. 



Serous membranes are of two principal kinds: 1st. Those which line 

 visceral cavities the arachnoid, pericardium, pleurae, peritoneum, and 

 tunicce vaginales. 2d. The synovlal membranes lining the joints, and 

 the sheaths of tendons and ligaments, with which, also, are usually in- 

 cluded the synovial bursce, or bursce mucosce, whether these be subcuta- 

 neous, or situated beneath tendons and glide over bones. 



The serous membranes form closed sacs, and exist wherever the free 

 surfaces of viscera come into contact with each other or lie in cavities 

 unattached to surrounding parts. The viscera invested by a serous 

 membrane are, as it were, pressed into the shut sac which it forms, 

 carrying before them a portion of the membrane, which serves as their 

 investment. To the law that serous membranes form shut sacs, there is, 

 in the human subject, one exception, viz. : the opening of the Fallopian 

 tubes into the abdominal cavity an arrangement which exists in man 

 and all Vertebrata, with the exception of a few fishes. 



Functions. The principal purpose of the serous and synovial mem- 

 branes is to furnish a smooth, moist surface, to facilitate the movements 

 of the invested organ, and to prevent the injurious effects of friction. 



