Animal Morphology. 83 



From mouth to anus there is one continuous membrane, 

 the mucous. 



Its- chief distinction is, the very varied forms in which 

 it is found to infold itself as inflections, constituting the 

 chief membrane, with connective tissue, of gland structure, 

 as salivary glands, gastric follicles, intestinal glands, and 

 the biliary and pancreatic glands. In all these there is a 

 great proneness to cell destruction and replacement in their 

 active functions. 



Externally the mucous membrane has its own serous 

 membrane, the peritoneum, which does not in all points 

 oppose itself as a counter membrane, but is deficient in 

 the buccal and oesophageal and rectal regions ; but it 

 follows it pretty closely in its abdominal connections, 

 to carry out its mechanical or passive function, as a 

 pliant smooth agent, in forming an extensive ^ and ever 

 changing soft joint. Here, then, the function is rather 

 vital mechanism than chemico-vital or alimentative. 



Between these two membranes is a third membrane, 

 possessed of contractile motion namely, muscle. At either 

 end, by a law of displacement, where function requires 

 it, the organic muscle gives way to striped muscle, as in 

 the buccal, palatal, and pharyngeal regions, and in the 

 rectal region likewise. 



Secondly, the lungs are examples of the tripartite 

 membrane. 



The mucous membrane from the larynx, ramifying into 

 all the smaller bronchi, ends in sacs, each of which has a 

 sphincter, guarding its commencement, of organic muscular 

 fibre the sacs themselves, connected to each other, 

 chiefly by connective tissues, serve as surfaces for blood 

 vessels to ramify ; and on the external part the pleura 



t close the whole, and, like to the peritoneum, serve the 

 irpose of a soft and pliant joint. 

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